While I understand the argument for limited skill bar usage for balance, I think I would personally dislike that approach quite a bit over time. I have no problems if they want to constrain the number of available spells at any point, I just don't want it to come via a mechanism of dragging and dropping spells to and from slots all the time. It's like reverting to inventory management using physical icon arrangement rather than a weight or a number of item limit. It's a tedious way to implement it.
