In a lot of instances, black steelies are more durable than cast alloys. E.g., cast aluminum can shatter like glass if you hit a rock the right way and will be irreparable. A steel wheel will bend. If you're in the middle of nowhere, then you can limp on a bent wheel.
BTW, where did you find good prices on black steelies? I've been looking for them in the 17" locally, and they are proving impossible to find.