Mopping doesn't help moisture retension but is used to add layers of flavor on meat. Each time you apply it (needs to have sugar in some form) it becomes part of bark.
Moisture is the function of finish temps and techniques.
I do sirloin tip roasts that have become a favorite for large caterings, I use mopping technique to flavor the bark from the start to finish (about every 45 min) other than sirloins I do mop much at all.
Jim