Sorry to engage in a bit of thread necromancy but how much worth does Destruction Dual-Casting have WITHOUT taking Impact, I have very, very few perks to spare (trying to make a jack-of-all-schools mage), I've heard that Impact makes all combat unenjoyable, is that actually the case?

It can, on one-on-one battles, at least. You simply stunlock your enemies until they die. This is true even for dragons, unless you use certain mods.

Yeah, I heard it made fights repetitive, that's why I've stayed away from it for the time being. But, is the Dual-Cast perk not worth it without Impact?

Well, the impact question is a matter of style but that's the only reason to take dual casting. Dual casting by itself just ups your damage 10% but at a 40% increase in magicka - not worth it except as a gateway to impact.

Worth noting that mentioned damage and MP cost increase is per hand.
Total damage increase is 20% and total MP cost increase is 80%.
Overall, not worth it at all without Impact perk.

Most definitely. Even without the Impact perk, it makes your spells noticeably stronger.
Edit: regarding the magicka consumption ... It's not noticeable if you level up your magicka attribute enough and invest points in the 'novice destruction', 'apprentice destruction’ etc. perks.

Wait, there are people who DON'T take those perks?!

^ Well if you plan to craft 0% gear, anything beyond novice is a completrly wasted perk.

Dual casting increases your DPS whether or not Impact is taken, so it does have some slight merit on its own.

I don't plan on using Enchanting, Smithing (or atleast no openly abuse it), or Alchemy.

With dual casting perk, you can make a stronger single spell with both hands which makes more damage (~ 20%) than regular dual spells combined. Also with Impact perk, dual casting spell has a stun effect which sadly works even against dragons, giants and other big targets. It is like playing with cheats. I agree that mages should have more defensive spells, but that stun effect should have cooldown or something. Random 50% chance or something pretty much would make it less useful because when you really need the stun, it won't trigger. Luck based defense is not a good idea.

However, dual casting spell eats a lot more magicka per damage point, but chain potion drinking with ridiculous magicka consumption reduction enchantments makes every fight so easy. Bethesda probably thought that high magicka consumption should balance the stunlock ability. Maybe if potions would have cooldowns and zero cost spells wouldn't exist, stunlocking would be very costly and impossible to maintain.

It's good only if you plan to use the impact perk (the next one on the skill tree). The 10% increase in damage is not worth the large increase in magica consumption.

But with the impact perk, many enemies will remain stun locked for the whole time you cast an offensive spell on them making some hard fights with damage sponges quite easier.