A few people are referring to abuse, but it's the built in AI here. If you know of a player abusing these abilities, however, we always encourage you to report it to a DM or the CC. And in no way, Balorvale, does this annoying situation with the game's AI or the HIPS system make using exploits acceptable. Ever. Two wrongs, and all that.
The dev team tries to adjust the AI and other systems where we can, but crying abuse or getting angry about it won't help us get to solutions any faster. If there's a way to make the experience better, we'll work at it. In some cases there are limits to what we can do and in no way do we intend to abuse systems or to hurt your experiences when playing.
I was using this an an example to stop players who have acess to this feat from abusing it, there is nothing I can do about the AI, but the point is, is that the AI should have a cooldown for this ability, perhaps a ten second window before they can HiPS again. It was very frusterating watching Purist and our entire group get trolled by one Vampire elite fading out and back out and back, switching targets and for us no to be able to react at all because she is spamming the non-target ability of HiPS, which is causing the frustration.
I bring up the counterspell thing as another example of a mechanic that is not allowed to be used, this HiPS thing, although not as severe as the counterspell bug, is still a large concern in that it makes normal fights incorridgable for the people taking point, not only does it pull you out of your defense, but woe to you if you clicked to open your inventory to check something while they vanished, (I usually find my closest Health potion just in case), as they vanish and then punish you with tons of damage, it doesn't contribute to anything other then making the enemy obnoxious, and I highly HIGHLY discourage all players from EVER spamming this ability, you WILL get in trouble, don't do it.
I suggest a cooldown, that's my two-cents, I'm not sure if it can be done, but if it can I believe it should for just plain common sense.