Simply amazed at how badly you're missing the point. No-one has said Conjuration, Alteration or Illusion are underpowered. The issue is that Destruction, as a DIRECT-damage magic school, is vastly underpowered by comparison to the other (non-magic) DIRECT-damage skills - namely, Archery, 1-handed and 2-handed weapons. Completely ignore support magic for a moment, and consider the following scenario.
Character A has full, perked Heavy Armour. Therefore he is defensively in good shape. For attack, he uses a fully-perked battleaxe, with +damage enchantments.
Character B has full, perked Heavy Armour. Therefore he is defensively in good shape. For attack, he uses Destruction magic, with -magicka cost enchantments.
Character A will do a HUGE amount more damage than Character B. This is cold, simple and pure fact. Read the following:
THIS is why people are complaining. Not because mages are underpowered, because as you say, they can complement their playstyle with several other useful skills, like Conjuration, Sneak, One-handed, Archery, with whatever the hell they want. The ISSUE, and the reason the game is unbalanced in favour of melee and archery, is that Destruction does FAR LESS damage than the other DIRECT damage skills in the game. Whether or not other spell schools make up the difference is completely irrelevant, seeing as any character, mage, warrior or thief, can also take those skills. The simple fact is that Destruction falls far behind as a direct-damage skill; for that reason, if you want to play as a pure mage, you are FORCED to focus on indirect damage (see: Conjuration) or to stop being a pure mage by focussing on other means of direct damage (see: Archery, melee weapons).
The (very simple) solution is as follows:
The extra "exploitative" damage achievable through Enchanting and Smithing could be balanced by introducing +Destruction damage/spell effectiveness/duration enchantments, which could then be stacked to make Destruction have the same sort of damage potential as Archery or melee.
Finally, please don't argue "I've played as a level 50 mage and I'm fine, so you're all wrong." This is a completely moot point unless you've also played as a level 50 warrior or assassin, at which point you would realise how much more powerful melee and Archery are.
I see the point you are trying to make, but it is incorrect. You explained it your self. Warriors have three ways to deal damage: one handed, two handed, and archery. In my opinion, you would only ever choose either one handed or two handed, but I guess you could use both. The same goes for assassin with one handed and archery, thought you could count alchemy and sneak because they allow you to do more damage. Now with warrior or assassins, their stamina goes down as they use their weapons. However, you can still use melee weapons to limited effect even with no stamina left. For archers, you cannot fight without arrows, but you can easily control how many arrows you want to bring with you, whereas a mage has to level up ton increase his magicka.
A mage has three ways of doing damage: destruction, conjuration, and illusion. His magicka level decreases with every spell use. However, he cannot attack at all when his magicka runs out. To compensate, the amount of magicka used per skill decreases as that skill increases. This makes it necessary for the mage to use all three skills in order to win. He is being given the ability to use more and more magicka as he levels up, so he needs to capitalize on that skill. If he continues to focus on a single skill (I.E. destruction), he will fail because the damage does not increase, the magicka does.
There are two different systems for two different playstyles.