Beat me to it. It's especially bad when you're playing a mage. Unless you want to risk the chance of killing your follower, you simply cannot use those spells. A perk in the Destruction tree would have taken care of this nicely: "Area Spells Harm Only Enemies". It doesn't seem to make much sense that an area Illusion spell like Pacify won't calm your followers, but your area destruction spells can kill them.
Indeed. Usually I don't have this problem with Erik the Slayer--we're pretty good at staying out of each other's way. Mostly because he's busy tanking the enemies while I'm skipping around behind them with my spells and a bit of fancy axework. (He's also decently sneaky for a giant Nord...still not the greatest of course, but Leeroy Jenkins he isn't.) Where I have trouble is in the really difficult fights where an epic spell that happens to be AoE would be really useful...but I can't use it, because I'd kill Erik.
I ran into this with the Red Eagle quest. The boss at the end of that is one mean [censored] at level 26--he'd knock Erik out in 2-3 hits and then come charging after my squishy mage. Given that she's a squishy mage, you can guess how well that would end.

I found I could cut his health in half with a well placed scroll of fireball...but of course deploying that would instantly kill Erik. I couldn't even Shout in that fight (much) since Shouts have such a wide blast radius and, well, one FUS RO DAH is enough to off a follower if they're down to that last sliver of health.
Finally managed that one with a well-timed scroll of Guardian Circle, a lot of running in circles flailing and spamming firebolts, and cowering behind Erik and casting Healing Hands on him constantly. Not fun times.
Sigh. I understand the logic behind having friendly NPCs also harmed by AoE spells, but a perk that nullifies or at least nerfs that damage would come in really, really handy for us mages who like to use tank followers. I don't blame Erik for running in ahead of me; that's what I hired him for. I blame the programming that allows for friendly fire.
