He is incredibly slow, that is the problem. I have fought one, and while the skill can definitely be game changing, he has to be alive to use it, and more often than not he will be dead before it goes off.
Also, it is still a 30% chance to proc. So what you you rather have, a 30% chance to proc and pick the skill that you want to proc, or a 30% chance to randomly get a skill from the enemy?
It is an interesting addition, but we will have to see it get more widespread to see how it actually shakes out.