Why would they take it for looks? It'd be for better protection since they're more easily able to fight on the front lines before damage gets bad.
That said, I'm thinking one person in ZD is stronger than Yama. My vote goes to Bonze, just because he comes across as the leader, if unofficially. But it's not necessarily the case. After all, 4 Yamas could probably take on the rest of the G13 with ease, so for these 5 to be the stronger force, none of them actually have to be quite at Yama's level (but somewhat close I'd say).
Well, as things stand yamamoto was actually stronger than the 12 other captains combined at any given time. To the likes of yamamoto captain level people were by all intents and purposes fodder (evidence by yamamoto seemingly defeating 3 stern rittern with no effort while in shikai). Even aizen was supposedly stronger than his 10 espada combined and defeated 8 captain level fighters with relative ease. With yamamoto being that high above everyone else in the gotei 13 then all the royals need to actually be stronger than the gotei 13 is to be able to take on yamamoto as a group. If the 5 of them combined can actually defeat yamamoto then the claims can still lay as true.
Yamamoto may or may not be stronger than the Royal Guards. The author is pretty liberal on retconning Bleach. But, at this point (assuming that the author doesn't retcon anything), Yamamoto lacks the qualification of being invited... aka having contributed something significant (which seems to be required to be a technological sort, not "social" accomplishments like setting up organizations).
As far as we know, Yamamoto (as strong as he may have been) hasn't invented anything.
^Yamamoto invented badassery on a scale worthy of the king of souls. Anyways, yamamoto might not have invented something like zampakuto or mod souls however he is someone to whom aizen refered to as "the history of soul society itself". One was or another everything SS is or isn't is to a large degree because yamamoto made it so through sheer badassery. We are talking about a guy who founded the academy, later on the squads as we know them and perhaps even started the military organization of SS as a whole. He didn't invent zampakuto, healing ponds or mod souls but he build SS as it is today (which is presumably better than what it was before him). The databook or kubo did say that the reason he did not get promoted to royal guard is because there is no one to replace him though.
Each member of the Royal Guard serves a purpose that goes beyond their strength. Yama for all intents and purposes seems older than all of them. His claim that no Shinigami had been born stronger than him likely still stands.
Fact of the matter is the totality of their power exceeds his (as it should IMO), but individually they are not stronger than him.
And while it may be more important to protect the King, Yama did that by proxy for the guards up until now. The fact is, no one to our knowledge has ever gotten to the King and that is a product of Yama and his baddessness, not the Royal Guards. It is likely that Yama existence as the protector of SS pre-dated there ever being Royal Guards, meaning there had to be some event that led to their creation as a back up plan in case he lost IMO.
Yammomoto will rise again from the Ashes to prove he is the strongest..
Can't let him go..
We still do not know if there was a Royal Guard or Royal Guards 1000 years ago. If yes, then there are individuals older than Yamamoto and could be stronger than him.
What makes the Royal Guards powerful is the fact that they have the Ouken imprinted on them by the King himself. Potentially this grants them additional powers which we do not yet know. Since the CC at one point said that the Key is passed from CC to CC it is highly likely that he possesses similar power (the Ouken) to the RG but probably not greater. So I would say that he is probably their equal in power alone, however due to all 5 Royal Guards being at this (Yamamoto's) level in total their power is greater than the Gotei 13. I would exclude Osho from this because he seems to be the leader of the Royal Guards and thus maybe be the second most powerful in the Bleachverse after the King.
Well, with his Bankai Yama was alone stronger than entire Gotei.
So now we should understand if 5 Royal Guards are stronger than Gotei including Yama or not. And that's a very interesting question.
Actually, I doubt that every member of Royal Guard is on Yama's level. I think Baldy is stronger than Yama, while Kirinji and Nimaya might be at Aizen's level, so a bit weaker individually than Yama, but still individually stronger than the most part of Gotei Captains combined.
And then I think go two female members of Royal Guard who should be around Shunsui's power level or maybe even weaker.
The question is how good are their Zanpaktous, since it's quite a considerable boost.
Personally I don't see Yamammoto being stronger then the royal guard, the classic oldman leggend being shown to be surpassed by youngsters.In the end, what did he achieve? he beat a mindless child, he had to use bankai to beat 1 Sternritter and his flames didn't burn to death a single being worth mentiong till now.Not even Hallibel's fracciòn, seriously....
Most of yamamoto's actual achievements unfortunately did occur offscreen. Yamamoto has been called the history of SS and it makes sense. He started the gotei 13, he defeated vandenreich 1000 years ago without using his full power, he founded the academy and he held the position of captain commander for 1000 years solely because replacing him was out of the question. Even shunsui is not even a mediocre replacement for the man... The only thing shunsui can really bring in right now is not his overwhelming strength (his strength and that of any of his collegues is underwhelming at most considering the role he has to fulfill) but rather a "HOLY CRAP WE ARE ABOUT TO GET STEPPED ON LIKE FODDER" mode so to speak and do every atrocity possible and go to every conceivable extreme to avoid getting foderized. Sending unohana to her death so as to make kenpachi enough of an asset to potentially replace unohana combined with the strength of kenpachi before his awakening is a damn good first start. Letting the other captains go about their business to train is risky but also necessary if it grants them a shred of extra power which they desperately lack. I do wonder what else shunsui is about to do to win the war.... Perhaps recruiting the remaining arrancar at HM? It would be a good chance to instantly get powerful militia instantly and avoid the soldat thing. Recruiting the remaining vizards is a must too.
And to be fair, yamamoto did do something like that 1000 years ago. The implication so far is that unohana was a horrible criminal, the worst one in the entire history of SS, so it makes sense to assume yamamoto recruited a bunch of murderers 1000 years ago so as to deal with vandenreich. Funny thing how the invasion seems to have brought out the best SS had to offer even when loosing yamamoto should have cost them 50% of their entire military might. Shunsui being in survival mode so to speak might just turn things around for them. And to boot ichigo became an actual shinigami and has manifested quincy powers in an unprecedented manner (through an asauchi).
As for yamamoto being stronger than the royals.... its hard to say if yamamoto would be individually stronger than the 5 of them on his own but I would think it is pretty certain that he is stronger than any one of them individually. Driscol was a captain level quincy (or at least strong enough to give shuhei the fodder treatment and deal a mortal blow to sasakibe) and even then he did not last 2 full seconds against yamamoto. Royd defeated kenpachi easily and even then he was not much of a threat to yamamoto. Allon was arguably reasonably strong and he lasted just about as much as driscol against yamamoto. If any member of the royal guard actually were on yamamoto's league I could not imagine how vandenreich would hold a chance to win here. It would be too much IMO. So IMO the 5 of them combined are at best as strong as yamamoto was which is already quite enough to put them ahead of the gotei 13 and more.
I agree that having five royal guards stronger than Yamamoto makes the forces of SS too powerful. I really doubt captains who served as his subordinates are stronger than him which seems to be Kirinji, Shutara and Hikifune at the moment. I think Osho and Nimaiya have the best chances to be as strong or maybe stronger. Yamamoto was most likely more than a match for the rest of Gotei 13, well at least when Kenpachi's power was manageable, in that sense having a combined power stronger than Gotei 13 actually implies that some, perhaps all of the royal guards can be taken care of quite easily by Gotei 13 if they fight on their own. I guess having an ouken on their bones possibly makes them more powerful than their earlier selves, it explains why five of them are stronger than thirteen captains of the Gotei 13 since making great contributions to SS is a more important factor than power in the selection process of royal guards.