Monster summoning's pet has a duration, it depops after it kills the mob you cast it on. If you cast it without something targeted, the pet will just follow you around and not do anything until you camp or zone, when it depops. You can use as many of the monster summoning pets at a time as you can cast, by 65 you have 4 levels of the spell, so you can get 5 pets + aa pet swarm going at the same time. Fun times on big boss fights w/out ramp or ae.
The pet can be useful for ghetto CC as well, although I don't think you can heal it. I also think that the pets count as ramp/aux tanks, so if you're up against a heavy hitter chaining the MS pets can provide some bonus mitigation for your tank along with their dps.