Well, actually, it is quite common for Pokes of a certain type to have a move that covers its weakness(es). An example of which is that dark Poke you mentioned. Though not always the case, battlers will have a move that hurts its weak-type, making it all the more versatile.
Example... The ever-popular OU Dugtrio. HP [Bug] will most likely be in its moveset since it works well against grass-types that it is weak against. Also, since Duggy is popular for going against OU Celebi who has a double-weak to Bug-type attacks.
Sacrificing STAB for versatility. It's a tactic that a lot of people will resort to for surprising and sneaky damage-doing. So it's not as unexpected as you might think.
Here's a fun example:
Slowking
- Earthquake (against electric)
- Fire Blast (against grass)
- Brick Break (against dark)
- Psychic / Surf (STAB, obviously.)
Refrain from using Ground- and Fighting-type moves in a single moveset as they are strong against similiar types (excluding Dark and Electric), but this is just an example, so it's okay.