Added 4 minutes later
I'm saying that you can achieve a general effect of reducing defense by -11, and even that's with a horrified scream from Daedra. And if her troops have +12 defense? Then there's still +1 remaining. In my opinion, having a full spellbook is much better than uselessly trying to reduce the opponent's defense.
Added 1 minute later
And I know about the range from -5 to +5; the rest is just "in reserve"...