HDs might be a special case as you can't tell when they break down. You are always in a position where you might need one tomorrow. Except if you keep piles of

HDs might be a special case as you can't tell when they break down. You are always in a position where you might need one tomorrow. Except if you keep piles of spare, but as HDs used to get cheaper constantly, that'not a common strategy for price sensible buyers.