It's problem as others stated is that it has little to no scaling options compared to any kind of weapon use. It doesn't matter how many times you can cast something when its going to take well over 20-30 casts to kill anything never mind a boss type. The other problem is how bad magicka regen is in combat.
Playing on Normal, level 49, never cast a conjuration spell other than soul steal (or whatever it's called). Never swung a melee weapon or used a bow. Destro all the way. There are ABSOLUTELY no enemies that take 20-30 casts. I don't even have max level destro yet (99! one...more...point). Dragons take 6-10 level 4 spells of opposite type, Named Dragon Priests, 10 or so. Everything else, WAY WAY less...Most "tough" mobs (vampire masters, storm elementals, named monsters/undead, etc) take 2-4 casts...Anything that isn't a "hard" monster 1-2, tops...usually 1. 90% of all player-race-NPCs take 1 spell, even the named ones. I enchanted my armor to 100% cost reduction more out of convenience, simply because I got tired of carrying around mana pots (which are SUPER cheap and EVERYWHERE) But even if I couldn't do that, I can kill almost anything without running out of mana (basically Dragon Priests are the only exception...Dragons take more casts than I have mana, but they swoop around a lot, aka, mana recharge time). And in those situations where I had to kill more than 1 tough thing before I got my 100% cost reduction, I just...GASP...drank some mana pots...DOUBLE GASP! You know, like every player character wizard in every video game ever is expected to do...
Is this different on super-duper-drive-you-insane hard mode? I am sure it is (no actual experience). Is dual wielding (or even just melee in general) better at dealing damage? I am sure it is (no actual experience). Does it matter? Not in the least. (Unless maybe your playing on the aforementioned hard mode, and you expect the game to be super easy, which I think is a lot of peoples problems).
My "fix" to the problem of destro being "under-powered?" Nerf melee...If you all can pull the sort of damage you say you can pull with melee weapons, then obviously the problem is that you are doing too much damage, not that destro is doing too little and needs to be brought into line with melee...
If you want the game to be super easy, play on super easy. If you want a real challenge, play on the hardest mode possible. If you want to play a mage, and it makes the game even harder, cause you aren't using the completely silly broken parts of the game, well then, guess what, you just made the game more challenging, which is what you're trying to accomplish by playing super hard mode, isn't it? So stop complaining.
That is all...