I am undecided on the potential of such a thing happening, if it would do bad only, good only, or both to some extent. It's a tough one to analyze, but I have the feeling that if it does happen then it's because Microsoft are absolutely certain of something concerning the XBOX 360's successor, perhaps they are certain of its success, but that's an epic gamble (with Sony, their main competitor, but also with Nintendo) which I think should be the one platform to benefit (general term) from such a move, simply because I don't see EA focusing only on PC gaming, they largely prefer the home console market that's for sure. If Microsoft can monopolize EA to work only on their next generation console along with making games for Games for Windows Live... then I seriously don't know what to think.
I see two possibilities at first glance:
1) Microsoft condemn themselves to generally mediocre games by monopolizing arguably the worst video games developer on the market as of now, paving the way to entire success for both Sony and Nintendo since EA is now gone on those platforms and now better games emerge for them, also giving more opportunities for smaller development studios and not risking to get over-shadowed by the behemoth.
2) Microsoft somehow miraculously "change" EA's game development methods and philosophies and end up being a large part in the re-creation of a "new EA", one that would regularly develop quality products, which would help convince consumers to buy "Microsoft only" (for their next console but might also help increase GFWL's popularity and usage frequency) and thus having more cash flow in (the ultimate reason, that is) by selling most of the blockbuster titles, somewhat leaving Sony and Nintendo in its dust trail at least for certain game types, mostly sports titles.
They're both extremes of possibilities, nothing of all that could happen, it's just rumors, but it's fun to speculate on this one in particular, I like it, just for what it could imply, just thinking about it... I mean, Microsoft, buying/merging EA... that's almost Hollywood-worthy. I wonder if that would be the biggest transaction in the video gaming history?