According to Diabloii.Net, the Necromancer will be removed from the Diablo 3 closed beta on PTR later today. While no specific reason has been given for the class's removal, it could be in reaction to mixed reception by the current crop of testers.
As an update to the first point, we will be taking down the Necromancer class tomorrow, April 18 at 10:00 a.m. PDT. After this time, we fully encourage you to explore and test some of our other Patch 2.6.0 features, such as Challenge Rifts and the new zones that have been added to Act II and IV.
We’ll update this thread once the Necromancer is available for testing again.
While at first blush this seems "bad", it should be remembered that closed beta testing is designed to do precisely what it appears to have done: Gather information and reaction to make adjustments to better suit the content and to respond to player feedback.
We'll keep you posted as the story develops. You can read more at the link provided above.