Well, act of defiance was the words he used... Anyways, what pirate has not declared war to the world government in a sense? The very act of calling yourself a pirate is a crime.... Also, its not like the strawhats did those things alone. They had a small army of their own in ennies lobby, luffy had help from ivankov, inazuma, jinbe and crocodile (all of which were stronger than luffy IMO) and the only reason luffy actually got to be relevant during the war was because he had every relevant pirate there covering for him and garp wussed out of beating the crap out of luffy. Heck, the strong people around actually told luffy he was too weak to be in the front lines of that war....
I still think that luffy being the son of dragon is the only real deterrent from offering him such a position. It's not like ace was not offered such a position even though he was the son of roger though. All the other strawhats should be perfectly viable candidates at some point though.
Lets also consider the pirates of the sun. Their captain trashed majiroa and jinbe fought and defeated countless marines and he was still offered the position and even the nobles had to accept it.
These are all legitimate points, but I have to disagree when you say that "all pirates have declared war to the world government in a sense". You have to be careful when you're talking about a "declaration of war..." The very act of calling yourself a pirate may be a
crime, but it's quite different from an
active declaration of war. I think that very few pirates have actively declared war on the government, or were considered strong enough threats to the WG that they declared war on the pirates. Obviously, Dragon actively declared war on the government, one of the reasons that he's considered the most wanted criminal in the world. Gol D Roger probably never actively declared war on the WG, but his knowledge of the poneglyphs/blank history and his influence as Pirate King was enough of a threat to the WG that they essentially declared war on him. Whitebeard also actively declared war on the government to rescue Ace.
These were active declarations of war: after these events, there's no way that the WG would offer shikibukai positions to any of these men were they all still alive. In the same manner, Luffy has been considered to have declared war on the WG
twice (his burning of the Enies Lobby flag and his ringing of the bell 16 times at marineford), and I don't think the WG will ever offer him a position.
As you noted, however, Jinbe was offered a position even though he fought and defeated a ton of marines throughout his career, and his captain trashed the holy land. In this way, I think it is possible that other members of the crew are offered positions, but not while Luffy's still alive. It'd be foolish of the WG to offer a position to... say, Zoro or Sanji while their captain is still at large. There's no way that they would abandon Luffy to join the Shikibukai, and the WG would just lose face by offering a position that they know would be shot down immediately.
Also, when you brought up your Ace example, I don't think the WG was aware of his being the son of Gold Roger.
http://www.mangareader.net/103-17410-4/one-piece/chapter-552.html if you look here in the flashback, they say that they have no background information on where this new D. appeared from. If they had known he was the son of Roger, I highly doubt they would've offered him the position, since they considered his very existence to be a crime.