Well, if you're a power gamer shouldn't you max every skill in the game anyway? I mean, no matter how powerful destruction is or should be, a build that features everything would still be better in the end. Sorry but you're not really making any sense here.
If you can kill everything using two skills, then what matter is it that the third skill you choose is something less significant?
He's saying that the magicka you just spent casting thunderbolt or what have you, is 'practically' wasted.
1 - Destruction's only purpose is to do damage to enemies. This is true.
2 - Conjuration and illusion do damage to enemies indirectly, but also serve as crowd control, and as a meat shield. Also true.
3 - The damage conjuration and illusion can do, in ADDITION to its other rolls, out classes destruction. Anyone who's seen the twins in action knows this is true.
4 - So why use destruction if it doesn't even perform its roll as good as the others.
This isn't saying 'waaah destruction isn't uber', this is people saying, "destruction doesn't have enough power to justify using it in most circumstances."