The problem is, many people are on overloaded connections because they try using unlimited or automatic setings (the defaults) for BitTorrent.
They're uploading too fast for their connection and are always choking, so they cannot download fast at the same time.
Your upload bandwidth will just sit idle while as much as 50% of all the data packets from their computers to others get dropped/destroyed by overload conditions.
Worse, their monthly UPLOAD bandwidth limits may quickly get exceeded despite less than 1/2 their uploads actually geting TO someone.