If the mage character has 100% Fortify apparel enchants to negate the cost of spells in the schools they use, then casting a stronger spell with both hands is more beneficial, than single handed casts that don't cost more.

To your point though, I believe you are right as far as dual casting being optional. Destruction is a lot harder without dual casting and the impact perk - the damage can be supplimented with Fortify Destruction potions, but the stagger from the Impact perk cannot. (unless something else is used to stagger enemies)

I've built a 'master illusionist' several different ways. One way was all Illusion perks and dual casting to effect enemies. Another way was only perks on the right side (including Quiet Casting and Master of the Mind) and only using single casts with Fortify Illusion potions to raise the level of effect.

I actually prefer single casts with potions, if only because I'm a snap-invisibility addict. (dual casting, I've got to get invisibility back in either hand, and the enemy will briefly detect my character then)