i have had a pet that has maycast entangle. and it has never used the talent and i have had this pet going on 2 years or more. its a waste of a talent if it never castes.
I've found that you have to cast certain spells for this to trigger. For example, on my Balance character I have a pet with Maycast Weakness and Black Mantle. I almost always used Ra or Sandstorm and my pet never once used it's ability. By chance I happened to use a single enemy spell (like King Artorius or Scorpian or something) and found that it will cast them about 25-50% of the time and usually both when it does work. So in my case, I have to use a single attack in order for it to work. You may have a similar case, like maybe it only works with single enemies or if you yourself cast a dispel or something. Good luck.