There has always been controversy about degaussing phono cartridges for the reasons you state above Scotty. They are delicate instruments with a magnetic field and very small tolerances. The possibility of screwing them up far outweighs any suspected benefits IMO.
As for demagnitizing CD's or vinyl records, I've tried it and heard nothing different. Logically it would make sense that it shouldn't have any effect, but then we all know that logic isn't always correct in audio, and we also know there is a lot of snake oil out there too. YMMV.