This just got put into DotA recently and it's overall a good feature. When a game goes from 5v5 to 5v3 in the blink of an eye it's nice to have someone switch and make it a 4v4 again. The issue comes in from the money in DotA. The team with the leavers gets all the leaver items, plus the bonus gold since the leaver left, plus the bonus gold that trickles in automatically since it's split between people. So once they get the 4th person they have a natural leg up.
I don't know how it'd be exploited if it (like dota) needa 100% approval. You cant switch unless every single person in the game OKs it. Maybe I'm not thinking sinisterly enough right now =P
You speak the truth. Its not exploitable at all since every player in the game has to agree with it, if they don't, it doesn't happen, if they do, its not an exploit.
Should have been implemented from the start.