I've been avoiding using any weapons that banish "Summoned Daedra up to level XX" back to Oblivion, because I want the fire / frost / void salts they drop. I have been assuming that when they are banished, they just disappear - is this correct, or do they leave behind the same pile of ashes (with loot) that you get when killing them the regular way?
Summoned daedra disappear anyway when they die. You can't get fire salts from a summoned fire atronach, only from wild atronachs that leave a corpse.
So banishing summoned creatures does cause them to disappear, but they wouldn't have been able to drop anything anyway, so there's no reason to avoid doing so.