All of my dogs get glucosamine. And chondroitin, Vitamin E, and fish oil. I am fully aware that there are no studies proving that it 100% will help your dog. But I don't think it's true, either, to say that it will not/cannot help your dog. Vets prescribe it very often. I use it with the thought process that it isn't going to hurt my dog, so if there's even a slight chance it will help, why not? It's a cheap supplement. I'd rather throw in 2 scoops of a supplement that could possibly give my dog an added 6 months-2 years of quality living than not throw in the 2 scoops and be wondering if it would have helped.
As a side note, my dogs get blood work done 1-2 times a year and nothing has ever been amiss. My vet thinks that a combination of exercise and glucoasmine, chondroitin, vitamin E, and fish oil are the reason my 10 year old GP who had a CCL tear that was not treated is still getting around perfectly fine.
I do agree that even though I personally would supplement with glucosamine, the best thing you can do for your dog is keep her fit and lean and be cautious of how much she exercise and goes up and down stairs, jumps off of things or out of cars, etc until her growth plates are closed. Those things are proven to be beneficial whereas supplementing may or may not help her in the long run.