Mayday is not just to get help, it is to give you a get out of jail free card to do what you need to do. Landing a glider in some random spot doesn't really qualify unless you think there are people on the ground that can here you and get out of the way.
Example - you leave KBWI and your airplane catches on fire. You call "Mayday, I have a fire, returning to the field" and then you ignore the radio. It is the BWI tower's job to get everyone out of your way from then on. It doesn't matter if they can help you directly, they can get everyone else the **** out of the way.
Real life example: I had an engine die in low IFR over Pax NAS. I called in the Mayday and while working on a restart the Pax River controllers, presumably trying to keep the civilian airplane away from their expensive Navy planes, was trying to vector me to a nearby rural airport with one somewhat short runway and no precision approach, no fire trucks, and no on-field navaid (pre GPS days). I told Pax River to bite me more or less, I was landing there unless they wanted to run out and turn the VOR off. Whether they wanted to help or be annoying, I had the power at that point. Then we got the engine going and left
