This is because dual casting does not seem to affect the level of enemy they affect (correct me if I'm wrong on that btw).
The result is that dual cast lower level illusion spells can hit higher level enemies with a lower mana cost and do it at range without the 5 second charge up.
Let me give an example:
Let's compare the Master level Mayhem spell with the Adept level Frenzy spell. At its base, Mayhem affects level 25 enemies. The Rage perk adds 12 levels, while Kindred Mage adds 10. This means Mayhem affects up to level 47 when fully perked out. Pretty good, and that will affect most enemies, but there will certainly be ones above that. Meanwhile, Frenzy has a base level of 14. You then add the 22 from Rage and Kindred Mage and you get to 36. When you dual cast, the level is slight more than doubled. I don't know the exact number, so we will just say it's doubled. This means a dual cast Frenzy spell can affect up to level 72, much higher than Mayhem's level 47 cap.
But dual casting makes spells cost more. So maybe dual cast Frenzy costs more than Mayhem. Nope. I've read that dual casting either multiplies mana cost by 2.4 or 2.8. Let's just assume 2.8 in order to err on the side of making Mayhem look better. Frenzys base mana cost is 183. That means it would have a base cost of 512 when dual cast. Mayhem has a base cost of 868.
So Frenzy will affect enemies that are 25 levels higher, while costing 60% of the mana that Mayhem costs. It can also be cast at range (allowing for more stealth usage) and does not take 5 seconds to cast. It is CLEARLY a better spell.
But let's put it in the worst possible situation. That is, let's say neither Kindred Mage nor Animage apply to the given enemy. This means dual casting can't double those level boosts. In this case, Frenzy would affect up to level 52 (base of 14 plus 12 from Rage, all multiplied by 2). Meanwhile, Mayhem would only affect up to level 37 (base of 25 plus 12 from rage). Quite simply, Mayhem CANNOT hit the same level as a dual cast Frenzy spell and it will always cost more mana.
The hilarious thing is that even the NOVICE spell Fury is better than Mayhem most of the time. It has a base level of 6. Add 12 from Rage and 10 from Kindred Mage, and it hits level 28. Multiplied by 2 when dual cast, and it will hit level 56, higher than 47 for Mayhem with the same perks. Meanwhile, that dual cast spell will literally cost less than 20% of the mana of Mayhem. Yes, Fury will only hit a single target, but it is ranged, so you can stay out of sight and the one enemy you used Fury on will typically start a big fight anyways.
Even if you assume you're fighting non-people/animal enemies, Fury is still similarly effective to Mayhem. Dual cast, it will affect level 36 (base of 6 plus 12 from rage, multiplied by 2). Meanwhile, Mayhem affects up to level 37 in that case.
This same logic applies to ALL the master level illusion spells (ie. calm and fear as well). In fact, I chose the frenzy line because it is the one that would SEEM to most need the master level spell, since it is the only one without an expert level spell.
EDIT: I'm not sure if dual casting ALSO affects duration. It really isn't a big deal either way, but if it DOES affect duration, then Frenzy would also have a longer duration than Mayhem, while Fury would likely be the same duration.