Well, if is never done it in 6 years there would be like an inch of dust, lint, and dog hair on 'em.
Yep...and then when you did them you would be able to see the results! I obviously don't do mine often enough, because I always feel the main floor hallway looks much better...but trust me, it wouldn't take six years for you to notice a need. Darn dogs and toddlers.