I got mine done recently. I never actually had any problems with it, but wanted it done as a precaution after reading this forum.
My local Yam dealer also said there was nothing on the system about it, but was vaguely aware that others had expressed concerns about the nut and had it changed.
I didn't bother arguing about warranties with them. I picked up the part myself from Wemoto and just handed it over when I went in for service, asking them to fit it with locktight.
They told me there was no sign of any problems with the old one having come loose, and handed over the old part so I knew it had been done.
Much happier now it's been done - was weighing on my mind a little. For the sake of a tenner well worth it I reckon.
Andy