Of course walls have to be able to attack and accomplish other things. Consider Forretress. In any given match, I might force two dangerous CB or CScarf users to switch, lay down two layers of Toxic Spikes, Rapid Spin away Stealth Rock once, and then Explode, taking a Pokemon down with me. Wouldn't it be reasonable to say that I got a good return on Forretress? The point of defensive Pokemon is that they are lesser threats than the offensive Pokemon in general, but they can still be threats to many things and can force switches and inflict statuses and field effects that enfeeble your opposition and enable your sweepers to annihilate. Frequently, they have the ability to get kills themselves (though seldom to actually sweep), and there's always the clever play that can leave your foe with nothing that get past your tanks while you still have them, forcing a slow but really quite sure win. This is why people use the tanks they do and not Pokemon like Shuckle. It's because of what positive things they can accomplish other than "dying slowly".