How Bad Is It To Not Clean Your Dryer Vent Once a Year?

Many home maintenance tasks don't need frequent attention. Left undone, most yearly or semi-yearly tasks (like washing the curtains or wiping down the walls) won't cause anything more drastic than a bit of extra dirt or dust. Neglecting other jobs--such as changing air filters, fire alarm batteries, or appliance maintenance--can cause not just nuisances, but actual danger. So, where does cleaning your dryer vent fall?