people who don't have a road bike, people training on a track bike, people who want to prove they can do long rides without gears and of course people doing long track races.

It doesn't really matter though the positions you see people in may not hurt you if you do 20 3 mile rides a week but it's still bad. You'll be slower cause you can't breathe properly or use your muscles well and you won't have good control of the bike. It's possible that over a period of time it may cause injury as well even if there isn't immediate discomfort.