It's the opposite of RP? The idea is you go back to town, spend some time setting your arsenal/spellbook/whatever up and then head into the fortress with your pre-set abilities ready to go. That's almost the definition of how things like mages work in games like D&D. You don't just get to use a micro-set of abilities from your entire spellbook and then swap it up when you feel like it.
But no, swapping it up between packs of mobs...THAT makes sense in an RP setting.
I'm not going to argue the good/bad reasons of why this should or shouldn't work in this post. But come on, saying that swapping abilities between mobs makes MORE RP sense than going back to a safe zone and setting your stuff up is a bit silly.