As per our Ops manual, Eng failure is a MAY DAY call. The list also includes many others scenarios as well.
as per my understanding uttering MAYDAY Means now you are also time critical which would defeat the very purpose of the call if someone opted to jettison.
the only reason one would probably decide to reduce a/c weight either to Max landing or higher just enough to ensure adequate landing dist margins are avail at the Land ASAP airport.
I rather not get into back stage factors as to why the ops manual says MAYDAY for single engine, or climb gradient requirements of the approach you make or like some people say be extra safe and think of the tyres etc...
one thing would be certain that overweight landing and other relevant checklists will all be carried out in time before landing.