Dark/Ghost is actually a mediocre defensive type. No weaknesses is helpful, but little resistances means heavy blows will weather it easily. Ghost/Steel with Levitate or Poison/Steel, or in fact Electric/Steel, has the capability to resist all but a few move types.
Ghost/Steel is neutral to Dark, Electric, Ghost, Water. WEak to FIre and Ground, if not levitating, and resists the rest. Poison/Steel neutral to ELectric, Psychic, Water, weak double to Ground and one to Fire. Everything else is resisted.
In fact, Dragon/Steel is one of the most brilliant defensive types. Neutral only to Dragon, Fire, and Ice, and weak to Ground and fighting, it eats all other moves with ease. Of course, neutrality to Dragon and Ice isn't as useful las a resistance, as they're in Ubers.
By the way, shouldn't we be looking rawly at the typing and effectiveness, not the stereotypical stats that come with it?