I don't see this being useful in dueling, especially with the current spell system. I could see it being used by some people who use those same 20 spells or something, but can't fit them all in 2 hotbars, but again you can inventory cast. I wouldn't mind it if it was toggleable but I don't think it would be necessary either. I mean we had this option before and I dont recall many people complaining after it was removed.