Actually, the point of dual damage is that because only one damage type can be resisted, they're always more effective against enemies strong to one of their types than a pure type weapon.

Pure type weapon against resistant monsters: 25% damage
Split type weapon against resistant monsters: 62.5% damage

In return, split type weapons deal 125% instead of 150% on monsters weak to them. Also, split type-normal weapons deal 100% on monsters they're neither strong or weak to, whereas split type-type weapons (ap/sentenza) do 87.5% to the 2 families they're in the middle for

A split-type weapon's minimum damage is 50% of it's maximum damage. A pure-type weapon's minimum damage is 18.333...% of it's maximum damage

Anyways, the point is, split-type weapons are more useful against resistant enemies than pure-type.

As for forgetting to change weapons, that's why you have (at least) two weapon slots - you should make sure to never have weapons of only one type, especially if that type is piercing - gun puppies are everywhere. A point could be made for shadow/elemental in the jelly palace or elemental in fsc, but no strata is just one single monster type.

I could approve of something like a grace period for switching weapons at the start of the first floor of each strata, but changing how the damage system works isn't a good idea - the game was balanced around it.