He is a powerful ally and his is a redemption story. Hard to resist. I left him in that cell once because I believed he belonged there, but never felt "right" about it. If this was a real life choice, I would say he should be left in there and let the Darkspawn rip him apart as he ripped apart the family he murdered. But from a RP stance it is just missing out on good story arc that I feel will carry over to something important later on.
Plus my mage usually becomes an Arcane Warrior. IMO, that is the ultimate tank. No need for Alistair. Sten is my warrior.