That's a weak reason, just for the fact, that Rebecca won't ever be powerful enough to beat DD. So it doesn't really matter, if metaphorical, or not, as she is "just" the Vivi of the current Arc. She has a role to play, but that is more likely a political thing, instead of fighting the DD Crew. Or at least DD and his commanders.
Well, Luffy want's to assure, that BB and no other tournament winner gets the fruit. Why would he then, after the strawhats defeated DD crew and Burgess, eventually, simply give it away? As i said it, the fruit already is a memento to Ace, for Luffy.
The strawhats don't need the fruits power. Every member already has his own fighting style. And Luffy isn't thinking about his alliance, as he is not a commander, but rather a captain. He wouldn't ever use the fruit to gain military strength.
Of course, everybody expects someone to eat the fruit in the end, but to me it's not the most appealing idea. Why all this fuzz about Ace at first, when you're just gonna trash it afterwards.
And in the end, this whole topic is already sounding like Naruto, where nothing matters, except the next power up and the whole plot only advances though power ups ...