Maybe an "hybrid" solution could be the solution: We could have as many quickbars as we want for when exploring and questing but when the game enters combat mode, we'd be limited to a "combat quickbar" with maybe 8 skills and 2 items slots maximum.
So that way we'd have the practical UI and also the "I have to choose my skills carefully" part for combat.
I'm not sure what limiting qucikbar in combat would accomplish. Yuu can still use skills and items from inventory and skills bar any time. It's just more tedious. Your hybrid solution would require disabling inventory and skills bar when in combat. I'm not sure it's a good option.
What I'm trying to suggest here is that everything in the game should be easily accessible without micromanaging and tedious rummaging and therefore quickbar should accomodate more space.
