i think towers are strong enough honestly. here's something to consider:
MR items don't generally have a lot of MR. rather, they have some MR and some HP. the 500 hp from a banshees veil will scale with whatever armor you have to resist a turret. there's a reason mundo can tank turrets no problem with a spirit visage and warmogs ;)
armor items tend not to have hp, the exception being randuins. so if there was mixed damage then it would be the same issue except flipped, because MR tanks would get double resistances (hp and MR) to turrets while AR tanks would only get one resistance (armor)