Mystic pets are fine the way they are, they really do not need better summons. The only reason one might want them to change is if you don't know how to utilize each for their special skills. I imagine that we'll eventually get the half mp fix on here, for now just make due with what's given.
They've never taken hp to summon, only mp, and it's really not that much except for Cragglord. Cragg is extremely OP, even more so than a veno's Herc. They have 10k HP, and can handle a lot. Speaking as a 100 Sage Mystic on PW, 20 seconds is plenty of time to take down things. Sage Cragg's time extends to 23 seconds, we really don't need him to come out for a whole minute or two. The only problem is if you don't know how to utilize these pets for what they are. Mystic's aren't meant to tank, we're meant to DD and backup the other players.