Sounds like an excuse if you can't aim.
Sorry, but I think this idea is very bad.
Possibly more like a selective area of effect spell. If I remember right, in D&D (yes, I know TES isn't D&D
), magic missile starts off as low level with only a few projectiles, and gradually levels up to produce a swarm of weak missiles which, if the targeted enemy goes down before they've all hit, veer off to do low-level damage to remaining enemies.
That would be more interesting than a simple enemy-seeking uber-kill spell, and would look neat without being overpowered. Also, it could be implemented so that you'd have to aim when you fired the spell off in order to give it the initial target for it to seek (otherwise it'd just fizzle out or fire off as a regular line-of-sight fireball).