User blog comment:AYET/Strawhat Pirates Armada/@comment-4493594-20130507152135/@comment-4493594-20130508074352

I agree that allies are important. As Mihawk pointed out during Marine Ford, to unite friends and former foes alike to fight by your side as allies is a dangerous ability. And it is what separates Luffy from other pirates. Luffy will have gotten as far as he will because he managed and will continue to get others to help him. (I half expect Luffy to give an Oscar Award speech at some point to thank everyone who supported him...)

But at the end of the day, the Straw Hats fight their own battles. If others fight with them it's because they want to be part of the battle at hand for their own sake. I highly doubt Luffy would call his allies to a battle they have nothing to do with the way old man Whitebeard did.