Time Travel is guarded mostly due to the reasons discussed on the What Can't Happen page. However, there are some specific reasons why it is so closely guarded.
When you travel back in time, you have the original entity and your new entity. They are co-existing.
This means that it is technically possible to create an unlimited number of people by continually going back in time. If Voldemort had a Time Turner, he could create a hundred
Voldemorts or an army of Death Eaters. If this were to happen, Voldemort would be virtually unstoppable, which is why time travel is so closely guarded.
Ability to stop death
Although this may be seen as a good idea in the short term, if all deaths were stopped, the World would become too overpopulated. It's a very bad idea in the long run and is another reason why time travel is closely guarded.
If time travel was freely available, people would be able to cheat on tests and get out of doing homework. It would complicate things and mess too much with natural progression. It would also create the ability to make a perfect world and free the world from pain and suffering. If any of you have seen The Matrix, you'll know that the idea of a perfect world never works in the long run.
October 31, 2007 - J.K. Rowling updates her site saying that she does not approve of "companion books" or "encyclopedias" to her series and later that day files suit in a Manhattan court against Harry Potter Lexicon owner Steve Vander Ark.