Never really found them especially helpful, but they tend to soak up those early control cards, which is nice when you have a 7/1 lava destroyer in your hand. Also, the AI likes to spend quanta killing early creatures rather than summoning their own, so they are little decoys soaking up quanta and damage while dealing their own (maybe pitiful amounts of) damage.
I got a purple nymph like three times, wow they die fast, would be more useful if every AI didn't have a way to kill creatures.