I'm not going to refer to it as 'official' and 'non-official', because that's simply dodging the facts.
I don't see why there's even an opposing opinion. We can safely assume that everyone here has played on an illegal server at one stage, including those who now play on legal servers. However, it's not true the other way around. Not all (In fact, probably none) of the people who play on illegal servers have played on legal servers.
How can you form a valid opinion if you've only experienced one side of things? I played on many various illegal servers for two years, and moved onto legal servers some time later. I've been playing on legal servers for two years now, and will never go back. Here are some facts:
1) Private servers are illegal. Stop making retarded excuses.
2) Private servers are broken, it's kind of like playing a game in it's alpha stage with cheats on.
3) Private servers are heavily underpopulated. It's not the way it should be played.
4) Private servers make people erroneously believe they've experienced the game (Noticeable in this thread).
5) Official WoW is an experience - You can never understand it without playing it.
6) Tying in with point #5, official WoW is absolutely worth paying for, and you'd only be able to see why if you actually played it. I can see why you think it's not worth paying for when you play on private servers. Why would you want to pay for retarded crap?