The whole point of the grind in wow and the high end instances is the sense of achivement of completing them. Private servers remove the whole point of the game from it, as they give you characters you can kill the final bosses with on your own. Also its not your character, its a pre-made lvl 100 or whatever. WoW is basicly a team e-sport, its the virtual equivilant to playing baseball/football with the lads from down the road, you join up in a team and try and kill a boss. In private servers you don't have the guilds to support you and the community and also they usually as i said make you reiculously powerfull so you can do it by yourself, or you get a bunch of people who arn't commited to thier charcaters, or don't understand them or just don't care.
I only used a prviate server once, and that was with about 5 other freinds to show my freind who was unwilling to buy the game what the endgame was like by doing an instance with him, which in turn actually convicned him to buy it so. But it only worked because the rest of us had lvl80's of the class we were playing and so we knew what we were doing and we played properly and made sure we were the same power as a normal character.