I don't know why they are popular, they really are a terrible idea if you are going to move about a lot. [ plus, kind of old-fashioned, but Kaneki isn't the best person to talk about fashion considering the crap he wears sometimes. ]

Yes, there is the School for imPorts which has areas where imPorts can train among themselves. Personally, I do my own training in surrounding areas without people around. [ old buildings, forests, places away from civilization ]