1st, I like to say, I like the TES IV way better. In TES IV, one can make 3 different effects (aka spells) into one effect. In skyrim, they cut it down to two. Dual casting in a way to combo two spells into one. Flame and Frost, at the same time = one effect(read spell) or heal and soul trap, etc If one dual cast the same spell = a new powerful version of the same spell
I am not saying it is a perfect system, but dual cast is a way to combine the effects of spells in a *new* spell...
Also, some peeps still think one can not make a staff, but one can. To bad it is so well hidden....