It means...
You can be a warrior and use your weapon of choice the whole game. You don't need any other source of damage.
You can be an archer and use your bow of choice the entire game. You don't need any other sources of damage.
To be a destruction mage you NEED another source of damage OR you must level enchanting enough to get 0 spell cost and even then it's just spamming a spell that does very little damage. Conjuration is supposed to be a support skill, it dominates in this game. Illusion is supposed to be a support skill, it dominates in this game. Destruction is supposed to be a primary damage skill, it's actually a support skill.
You could play on a lower difficulty but that also means the bad guys also hit you for less damage. On master they hit for 2x damage, which means that if you get hit you are probably dead. That's a good thing by the way. Mages are supposed to be killing machines but also very easily killed. That's not the case in this game.
Depending on how you look at it I see one of two things 1. Destruction is underpowered or 2. Everything else is overpowered
In my opinion not only is Destruction not balanced with archery and melee, it's not even balanced within magic itself. Conjuration can be used and you don't need another damage skill, not even on master level. The same thing with Illusion, you frenzy the group and calm the last guy. Attack, calm, attack, calm. You could just punch him to death. Of course now you can't frenzy/calm every single thing in the game but my point is that most of the game becomes way too easy.
With Destruction there comes one great perk, impact, and that makes the game even worse. Instead of having to run around in circles throwing 30 spells at a guy I can just stand there and stagger him the whole time. Still not fun though.
See, I understand what you're saying, but there's something I still don't get:
Do you think my warrior is one-shotting every enemy he sees? No, he isn't. He's taking plenty of hits. He's blocking them with his shield, or absorbing them with his armor. Sure, I one shot low level enemies, but higher level and bosses? I give and take hits. I just have the ability to do that.
Archers/assasins are using stealth to get that bonus to their first hit - and to the next, as an archer, if the enemy still hasnt found them.
I don't think that using illusion/conjuration/alteration is such a bad thing. It's using complementary skills. You use them to deflect attention while dealing damage with your destruction spells.
I fully get that people think destruction spells don't deliver enough damage per use when facing higher level enemies. But nothing should be one-shotting EVERY enemy, even destruction spells. If it takes 2 or 3 hits to kill an enemy, why is that such a huge deal?