A controlled and focused blast of magic is far more deadly that rapidly slung spells whose magic has left the mage's reach as soon as it's slung.
I understand your logic, but the magic in Elderscrolls has never worked like that. It all depends on what's in the spell. Take a fireball spell, for example. In Morrowind the Firebite spell had a bigger bang as a purchased spell than Fireball, but one only had to buy a more powerful Fireball to do as much, if not more damage. Think of it as a nuclear bomb... whether it explodes next to you with the person hitting the detonator right there, or if it explodes next to you after being shot from another continent... it's the megatons in the bomb, not the initial proximity to it's intended target, that determines the damage taken.
Now there should be types of magic that could mask your attacks. Say... an fireball cloaking spell, but it still shouldn't change the damage of the spell, just hide your location.