While most summons do not carry anything of much value, a few can be exploited in this way to harvest their gear. There is a short period of time after a summoned creature is killed during which you can loot its corpse just before it vanishes.Those born under the Atronach birthsign can use summons such as the Ancestral Ghost to replenish their magicka, by summoning a creature, striking it three times, and then absorbing the spells it casts.
(You will not receive a soul if the summon spell expires, only if the summoned creature is killed.) Golden Saints are the best for this, as they have the largest souls available short of a few unique deities. Summoned creatures will disappear when the spell expires, when they are killed, or when the character who cast the spell is killed. Summons a creature to fight for the caster for a limited period of time.