Destruction is underpowered when compared to any other direct combat skill or when compared to Conjuration or Illusion. Ignoring armor is pointless when almost all the enemies in the game have dire armor ratings in the first place.
Dual wielding is a moot point, because any weapon dual wielded will make destruction look weak. Without exploits and by only using 2 perks you can easily smith weapons well beyond 150 damage unenchanted, which when dual wield means 300 dps. Thats a full 100 above destructions output per second. DPS is not the same as damage, as melee can have more attacks for more damage in the same amount of time as a dual cast or double cast spell.
Anyone arguing that destruction is not underpowered compared to other combat skills doesnt understand the game mechanics properly.
Firstly, you misused the word "dire." If enemies did, in fact, have dire armor ratings, Destruction would be an absolute must.
Secondly, I'm pretty sure that we here on the forum have a fine understanding of the game's mechanics, and probably better than yours, given your final statement.
Being a "Destruction Mage" and using only (or almost exclusively) Destruction are two different things, and will obviously be about as effective as using only an offensive combat skill and little else. Maybe people expect a lot out of mages, like for them to be able to effortlessly burn up a whole town or something, but in order to do that, we need to focus on more than just Destruction.
Finally, even considering Destruction and nothing else, (1) it's best compared to Archery, as both only deal damage at a distance, however small or great (and Archery is fairly comparable to the other two offensive combat skills, so that's a balanced way to look at it), and (2) magic can do unique things.
For instance, arrows cannot AOE. Even two handed weapons with Sweep cannot hope to compare to the basic fifteen
feet of AOE that Adept spells have, no less the great distance of Master spells. And while they can be poisoned to great effect, combat skills cannot DOT without supplement via poisons (which means Alchemy investment, gold investment, or luck finding these one-time use items).
Before even going on the offensive (and let's face it, that's all bows can do), Destruction magic can set up a spectacular defense. Walk into battle with a cloak spell and Ebonyflesh, cast both. Switch to a rune in one hand and a wall spell in the other, casting the rune in front of you in the direction of the opposition and circling the wall around you, and viola: hundreds of points in both defensive damage and armor rating before you even start attacking.
And none of that is to mention awesome things that magic is capable of doing, like Mass Paralysis and Blizzard together, or, say, completely
obliterating Ancient dragons in just 40 seconds with one spell (3,071 HP divided by 75 damage/sec, which is Lightning Storm
unimproved).