I'm not familiar with the specifics of the protocol, but your download speeds would be much slower without uploading. I believe that with BitTorrent, you are penalized/rewarded somehow for not uploading vs. uploading.
Not really, trackers will impose restrictions on users based on seeding ratios, but if you just DHT a random torrent whoever gets bandwidth is more down to random luck than priority towards other seeders. There is just no protocol level mechanism that prioritizes seeding to someone with a higher upload - the choice of who to seed to is up mostly to the implementation.
