The statistics are nice, but this is the key, I think:
Summoned trash also has a few other advantages. It can hold a line whether needed as a rearguard or instant bodyguards — fireballs do not protect squishy mages or buy time for a retreat. It is a beyond visual range weapon: you can summon stuff, send it into a room, and wait for the shooting to stop in order to soften up targets; very few spells do not have line-of-sight requirements so this is big. Conjured critters take attacks that would have otherwise been aimed at PCs, thus giving the spell defensive value as well. It is also useful for problem solving, tricks & traps, etc. Finally, summoned trash is discriminate; you can use it when a large mixed melee is going on and you don’t want to risk friendly fire.
UPDATE: So, do you play Monster Summoning as pulling existing creatures out of their normal place (and/or time) and popping them over to do your bidding? Or are they completely new creations whipped up by the spell? I’ve played it both ways in the past.