There are many reasons why, if your interested in playing WoW, you should play on the official servers.
1. Bugs GALORE are ever present in private servers. They are developed by hobbyists and there is no QA process for code changes and additions to go through.
2. It takes a long time for new content and changes to reach the private servers, once it does, a new wave of bugs normally come with it.
3. Capacity. The quality and efficiency of the code in private servers will never be of the same standard as Blizzards as they run benchmarks, have strict QA processes and are constantly refining and optimizing the code base. Private servers fold under the strain of a few raids running, they are significantly more demanding on hardware and as a result can only handle a tiny max population.
4. Frequent loss of characters and restarts. If you play on a private server, do NOT become attached to your characters. As private server development is done and new versions released, the developer may often require the admins to wipe the character base. Another thing that happens a lot, is a competitor releases a slightly more updated/superior server but with a different database structure. The admin decides to change over to use the new server but won't migrate the existing character data, which is understandable as it can be a heck of a lot of work, especially for one person.
The list goes on and on, but the author's original point probably describes it best, "You get what you pay for".