It should definitely provoke opportunity attacks. Standing up from prone does, and it preserves that aspect of game balance. More importantly, however, it just makes sense. You can't effectively guard yourself if you're pulling your buddy off his ass.
I also think you should specify that, if the ally has been subjected to a lasting prone condition (such as one which is caused by the unconscious condition or one which lasts until the victim saves) you can't pick him up, because he'll fall right back down again. But most prone effects just knock you down and let you stand back up again at your first opportunity.
But I like this. It's not always a great tactic, but sometimes it'll save your life. Prone can be surprisingly nasty.