I had knee pain on and off for ages, still get it when I run, took glucosamine, MSM, Chondriotin, cod liver oil etc. Didn't make the blindest bit of difference.
I was convinced my knees would just keep deteriorating and in the end I thought to hell with it - I'll have the use of them while I'm still young and if they're nailed when I'm older then so be it.
Now I ride more than I ever have, train 5 days a week and ride/race lots of big mile enduros, stretch lots and my knees have never been better. On a similar note I finally took the time to properly stretch my hamstrings and back recently (like an hour of stretching, a 2hr ride and another hour stretching) and was amazed how fluid my pedalling style on the road bike had become this week. Feels dead smooth and easy.
I'm not sure about the whole 'well it can't do any harm' argument - I mean you could send me some cash and I'll pray for your knees. I'm not religious like but it can't hurt...