Well, the new Dark Pokémon are always Darkness and their original type to show that they are Dark Pokémon.
Delta Species Pokémon were at first part Metal to show that they have been altered by the Holon Scientists (Haha, I'm so cool). Later on, more Delta Species Pokémon were discovered, which would probably explain the basic form single type, but most of the final forms are still part Metal.
I personally think the Dual Armor Poké-Body is a better option than forcing them to be two types. With Darkness/something and Metal/somethings, you get bonuses regardless of whether the target resists Metal (but nothing in the game resists Darkness?).
Let's say you've got that W/L hybrid Lanturn. There could be a time where you'd want it to just be its base type, say, Lightning, in order to avoid Resistance. Or maybe it doesn't fit in your deck if you use the Water sub-type, and if the Lanturn card is forced W/L, you're saddled with that Water type that could hurt more than it helps. If you're fighting something that resists Metal, at least you've got another type to nail weakness (No Grass Pokémon that's resistant to Water is weak to Lightning) and a Metal bonus. With normal types... you got nothing.