Inventory casting does give players more options, but it is balanced by the fact that you have to open your inventory in the middle of a duel, which leaves you vulnerable. It is also difficult to cast spells quickly from your inventory because you have to move your mouse precisely over the spell you want to cast, and if you try to do it quickly you can miss the spell you wanted.
I third hotbar would also not be useful to passive players because they can just switch spells out from their inventory into their hotbar, or inventory cast them at a slow pace.