Sterotypes are useful for a quick judgement. If some young guys approach me dressed in gangsta gear, I will be more defensive than if a group of elderly ladies approached me. I think this makes sense.
However, I should be willing to accept that my prejudice could be wrong. When it is not a matter of safety, I should try to err on the side of being proven wrong. I think prejudice is useful when we apply it to things that keep us safe. It is not so useful when we do not challenge our impressions.