I think the proper reply when your opponent calls a prompt let for a ball rolling around behind you is "Thank you!"
From my side of the court, it can be difficult to judge just how far behind you the ball is. While I am squinting and trying to see if the ball is close enough to you to be a threat, I am no longer focused on the point. So I call a let as soon as I see that ball, regardless of who was ahead in the point. Sometimes I cost myself a point, but it is worth it to avoid injury to my opponent.
-- Random Error Generator, Version 4.0
-- Master Moonballer