Apple knows that with open source they have outside contributors (And don't have to waste X much money on devs).
They reason they want to do it is because they're spending so much money on maintaining GCC ports, that they figured it'd be easier just to do that. That's why pretty much the entire project goal is portability, with minor speed goals and stuff.
I have no problems whatsoever with using it. In fact, TBH I'd rather use it than some crackpot named Richard Stallman's stuff (He thinks big brother is watching him at all times).