Rumbo,
By calling a Mayday or Pan gives you free airspace and therefore extra brain cells to fly the aircraft.
Its called load shedding. I would rather tell air tragic so that I know I will have the runway for me and everyone will get out my way. Also the R/t drops dramatically.
This has to be a good thing for low houred pilots in particular. They need all the brain power to fly the aircraft.
Something I see very frequently in Multi Training (Senecas) is that pilots come in high and fast on a simulated single engine approach. They are frightened to get "low" and "slow" on the approach (a good thing) but fail to achieve the correct approach angle. They then find themselves over the grass strip going at warp factor ten. If they had made a definate decision at commital they would have the option of go-around. But they very rarely do. They are hell bent on salvaging a balls-up approach.