Well my father in law has been seeking out small sized women for walking on his back for years. He really likes when I come over.
His back hasn't gotten any worse for wear. According to MIL, his back has been screwed up since DH was born...wonder what that means? LOL
Walking on his back- is one of the clearest memories I have of time spent with my dad when I was a child. He used to get me to try to really dig my toes in too... He is about 80 now- and no back problems!!!
I can't see is as any different from what you would pay a good sum of money to have a chiropractor do.
As others have said it can actually be good for your back, relieving pressure and pockets of air that can cause damage.
It can get detrimental if you over do it. I know some people that pop their knuckles daily, and really just spend all day twisting their fingers about, and things like that can cause the cartilage to wear down and degrade.