Shun the non-believer.
The vet may be right where dogs/cats are concerned, I have no idea.
But controlled studies seem to indicate some supplements help for people.
Here is one article from WebMD:
http://www.webmd.com/vitamins-lifest...de-glucosamine
Here is an article w/ more detail from the NIH:
http://nccam.nih.gov/research/results/gait/qa.htm