Nobody said you have to put your weapon away to cast magic, if you want to be a spellsword then you put "spell" in one hand and "sword" in the other. The class isn't called a swordspellsword or swordshieldspell.
I like the idea of how the hands work, seems more balanced. If you want two weapons, then you're going to have a more difficult time casting spells, that just makes sense to me. Hell, in Morrowind this wasn't possible AT ALL.
I couldn't agree more, I think the new system sounds great as you say it makes sense. Also the fact that you can use both hands for a more powerful spell is excellent.