I think there's a lot of allure for private servers beyond the free aspect. It'd be neat if you could legally play on a private server if you were paying a subscription fee to blizzard or whoever actually owns the game. There's a lot that can be done with a small and/or like minded server population that just flat out won't work on official servers. I wouldn't even mind paying an extra $2-3 per month for the right to play on private servers. But it would never happen, I'm sure there are some good reasons why as well.