Well, the ONLY magic-damage monsters in the Slime Pit have the resist-down property, so it's not like you'll have any other chance to actually utilize your resists. Uh... good luck by the way, you masochist.
Obviously if your resists have outlived their usefulness and there are no longer relevant monsters that match up to your resistance type, you can freely discard them. However, these are rare and exceptional cases and hardly the norm.
What a cruel idea! Tomithy would become one of the most dangerous and pesky normal bosses, and it would add quite a new level of challenge to Namtar, Matron, and the Avatar. Not to mention other places like VHoS or The Slime Pits.
In addition to your idea, you could up the resist down on most monsters, but make it cure on leveling up.