To ERR is human! Keeping 121.5 on means that if you dial in a wrong frequency when changed to (eg) Tower (by mistake) then there is a safety net available by having 121.5 on UNLESS it's being blocked by idiots (generally). A call to, or from said tower on 121.5 (for landing clearance) could prevent an accident/incident occurring.
My own experience says that most "blocking" of 121.5 actually comes from airline co-pilots inadvertently transmitting on the frequency when trying to contact handling agents and having failed to select and check their switches. Recently, in a two hour flight I heard three examples of this (I routinely listen to 121.5 on my secondary radio when it's not being used for obtaining ATIS etc, even though my present operation is not mandated to do so).