Letting the player choose gives the player options randomly summoning doesn't allow. For instance, I used Conjure Animals in ToA to summon Giant Owls to lead off the flying T-Rex, and apes in the dungeon to solve puzzles/ open obviously trapped chests. Randomly summoning stuff doesn't allow for options like that.

But this whole topic has gotten a bit out of hand. The player choosing has the potential to be the fastest way to handle the spells, but in the same way the DM shouldn't be a dick and give the player useless monsters, the player shouldn't be a dick and abuse the spell either. It's still a great spell if you have to downgrade to Boars over Velociraptors.