I imagine Ace might be upset about this because he holds Whitebeard as his adoptive father, and Roger was Whitebeard's enemy.
Some thoughts: Presently, there are two forces out to rescue Ace, Whitebeard's group and Luffy's group. Let's look at the map of the area:
It would seem that the crescent shape of Marineford almost faces the Gates of Justice, so if Luffy and co. manage to get through, they should first be facing the ships surrounding the island, then the Shichibukai standing at the front. That is, of course, if they manage to get through before Whitebeard arrives–-if not, the Shichibukai will undoubtedly be deployed against his main force. And Whitebeard should really be coming from behind or to the side of the island, since he certainly went through Fishman Island to get to Marineford, and given that Fishman Island is right below Marajoia, he may very well surface right in the middle of things. In any case, if he does surface before Luffy and co. get through, it will probably be for the best, since that will draw the Shichibukai away from the coast and make it much easier for them to sneak in---in the chaos of this war, it would probably be fairly easy for a single Marine ship to go unnoticed, even if the Marine's do have some warning.
But if Luffy gets through first, it presents some interesting issues. For one, what will Hancock do? On the one hand, betraying the WG would doom Amazon Lily, but on the other hand I doubt she has it in her to attack Luffy. Then there are Kuma and Mihawk, both of whom seem to have some connection to Dragon, which would make a meeting with Iva-san very interesting to say the least.
Speaking of Iva-san, one wonders what Crocodile will do if (and probably when) they join the battle. Will he go off to fight Whitebeard as he pleases, or will the threat of Iva revealing his secret keep him in check enough to help Luffy reach Ace?
As for the revelation about Ace itself---well, it's very interesting, but since it's not relevant to the action at the moment it feels a bit forced. I did have one thought about it: Portugas D. Rouge managed to hold Ace in for 15 months at the least. The fact that she died isn't surprising, but how did she manage it in the first place? Being a D., she probably had astounding willpower, but would that alone be enough? Maybe she had some help from a certain member of an anti-government faction, one whose hormones allow the human body to do otherwise impossible things...