I don't really understand why this matters (I honestly don't, not trying to be nasty). I'm saying that people who want to buy RE6, GTAV, or AC3 probably already have a 360 or PS3 where they could buy these games.
You are correct. The audiences are already established there and nobody outside of the extremely hardcore are going to be falling all over themselves to buy a new console for games that they can get on platforms that they already have.
But still, it's better to give it a shot and attempt building an audience than to not even try, right? It's not like they'll lose an arm and a leg just porting.
Do developers "gain much"? No, not really. Especially not in the short term. But I think they'll gain something regardless, even if it's a relatively small.
I'd imagine that that is a huge assumption to make.
Not really. Take the late port of Mass Effect 2 on the PS3 for example. Its sales weren't very impressive at all. And I'd bet anything that Mass Effect s's sales results are extremely skewed in favor of the 360 (I think the UK's results were something close to a 80/20 split). Why? Because most people interested in Mass Effect already got the console that the series was established on. The same thing would happen in your hypothetical situation of Halo and Uncharted ports. Wii U ports will fair no better either, probably worse.
Do I think it was dumb of Bioware to bring the series to the PS3? Of course not. I'm sure they gained some profit from it and expanded the audience of their franchise. I don't think that developers porting their games to Wii U are making stupid moves either. But how much the developers are actually going to gain from it isn't as significant as what you're assuming.