I've already used my tanking companion in several flash points after the tank/s left due to their severe lack of tanking gear. Pvp gear and less health than the healer doesn't get you a very far in hardmode flash points.
Using my tank pet works well for the most part, it's really about the same as playing a hunter in WoW. Except I'm healing instead of dpsing.
As for using my companion to tank all hardmode flashpoints full time. The controls and AI might need some adjustments. Right now I can't order my companion to reposition to X location while tanking Y mob. Changing the direction of facing is also a pain.