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. Stating anything BUT 121.5 should be left for unusual or emergency situations is pure lack or experience/understanding or professionalism.