Let's compare Summon Hydras to Summon Ugly Thing.
Summon Hydra:
+Higher Damage Spikes
-Extremely Brief Duration
Summon Ugly Thing:
+Easier to cast
+Longer Duration
+Possible Chance of Very Ugly Things with mean brands (red has Sticky flame!)
I frankly can't see why you'd choose hydras over uglies! Even the book it comes in is better (Book of Summonings v. Book of Beasts). Beasts might be cheaper if that's even remotely a factor, but uglies beat it by far.
It is not easy to see why you'd do it when you list them like this, where a + seems worth as much as any other. But you underestimate the higher damage, it is situationally much more important than the summon ugly thing advantages. An easy way to illustrate this: let one hydra fight against 3-5 slime creatures in a corridor, then let one ugly thing do it. This isn't just a slime creature thing, though; in every fight where terrain plays to a summoner's disadvantage (when it isn't very open) Summon Hydra is much much better. And it also kills single dangerous monsters quicker than ugly things. So keep both memorised, with the same reason as for memorising IMB and Bolt of Cold on a conjurer: one is usually better, but you need a spell for times when it isn't.