I think private server are good. I started playing on a private, just to learn how to play and learn all the classes. It was a fun server, so i basically made from 1 to lvl 90 in 1 day. Then i got banned, so i started playing official wow and i knew already what to do (race/faction/class/profess)
Yeah they can be good for some people, I don't really have an opinion on them honestly