I mean, a ponzi scheme would imply that my purchases are also making someone else rich. I did not "sign up" for bitcoin on some elses behalf. Thus, it is a free market, with price volatility. I think the more places that accept bitcoin as a payment will help the masses accept it as well. Plus, the value of bitcoin is just one of the many benefits of the technology. The real benefit is in the blockchain technology, and how that has made some many other functions a reality (storage, money transfer, etc). Coins are a by-product of the technology, in order to link economics with technology in the same thing. Why not?!