Like Angelus said, I wouldn't worry about it as long as it's a magnified optic. As long as it's not an über-cheap scope, the convex surface of the lens should make it resistent to BB hits. I've had my Elcan replica shot in the lens many times without issue. Th e protector caps will work as scratch protectors, but will break if shot.
Red dots / holosights though have very delicate front lenses (even real steel EOTech). With those, a protector is highly recommended. One unlucky shot and you're out of an optic.