I think it would compromise the quality of the story. Yes it would be nice to get all the doctor back together, but the quality of the story would be poor and as I said, it would be overcrowded.
Oh for goodness sake, you're making me sound like a mean bastard (ok I am a mean bastard, but... :P). I'm not saying that because they're old they should have nothing to do with doctor who ever again, I'm saying that they shouldn't all appear in the 5th anniversary episode, but they can do more to celebrate doctor who's 50th then appear in the episode. Interviews, comicons or whatever, sketch shows, competitions, appearing on themed quiz shows, bla bla bla.
Yes I know they are, and I'm thankful to them. If I met Collin Baker for example, I wouldn't put a bag over his head, through him in a van, shout "ok, old guy gone, continue as normal", and drive off, I'd probably go up to him, say hi, and maybe have a conversation. Really I'm not saying that all the old actors are worthless, I'm saying their time on the show is over.
Well I can't rally conceive of a way. Wait, you trust
Moffatt? Dear god, god save you.