I believe that both approaches are fully compatible. Imagine that you get that new spell (or decide it's moment for a change of strategy) and from that moment you want to start using a spell. What would you do with the current system? Go pick the "new" spell and set it as favourite. With the dynamic system you could so the same and force that spell to become favourite. As you would continue using that spell, the dynamic system would keep the spell as a favourite. And if you don't use the spell and the system removes it from the favourites, do you really want to have it there?
It's exactly the same number of clicks, with the advantage that the rest of the spells, the ones you don't manage actively, are managed by the game.