I guess illusion works wel if you wanted to play as an imperial Jesus. I mean it works if you don't want to fight, but a lot of perks are needed to keep it effective. And like I said I agree that weaker destruction spells are more effective than stronger ones.
Also I got up to lvl50 on my high elf mage and I pretty much only raised magica. Destruction was at a 94 and everything else was 90+ except restoration.
What are your perks and what gear are you wearing. Also, Illusion is much more useful for someone who wants to fight than someone who doesn't. My link just showed someone who got to 35 without fighting by using it a lot. Take my previous examples. When you frenzied or rallied one of the two enemies attacking you, you can attack both of them while they kill each other, then take out the last man standing. Same with causing an enemy to flee or be calm. You use the distraction to your advantage to attack without being attacked.
Edit: @Azuron: I understood what he meant, but I was just saying that playing a mage can make using the right gear more important than a different class. I agree that the magicka cost and damage don't even get close to scaling properly.
If you look at the spell Firebolt it does 25 damage for 41 magicka. That's .61 damage for every one magicka the spell costs. If you look at Fireball it's 40 damage for 133 magicka, which equates to .3 damage for every 1 point of magicka the spell costs. So yes, it does get weaker, and that makes no sense. Whether that is an oversight, bug, or intentional I cannot say.
As for toggling god mode with the 0% destruction spells, to get that you need Level 100 Enchanting (unless you use potions or exploits, then it can be lower), 8 specific Enchanting perks, and 4 Grand Souls. That also means that you use 4/6 armor enchanting slots (not including shields).