Where I save levels is illusion and conjuration. If I level those skills I'd expect to use them in combat. I haven't touched them and haven't picked up books on them or other skills I don't use because I want to use destruction as my sole damage dealing skill. How can you justify leveling combat skills and then not using them?

Edit: But to answer your question, the reason why you'd have more than one "damage dealer" is for added flexibility. If I see a mage that I can't get close to, I use my bow and fight him on relatively equal terms. If I see a crowd then I sneak and whittle them down a bit first. It's not so necessary now but it's saved my ass plenty earlier in the game.
And my plan with illusion is to let crowds whittle themselves down a bit first all by themself, in case I ever feel out of my league. Like if 10 deathlords start playing shoutball with me, I might want a different approach. Similarly, 10 or so briarhearts would be a bit much if they're all at once.

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