I don't know what the -5v rail is for. I'm thinking it a holdover from previous generation PSU that hasn't been elminated yet. Kinda like the floppy drive.
My BioStar mobo (MCD 400?? or something like that) says that the voltage on the -5v rail is -65v!!!!. I've had this mobo in my HTPC for two years and it has not imploded yet.