Bitcoin has a fixed supply in the long run, but if you look at nearly everything that's being used as actual money, you'll notice that it has an elastic supply, generally controlled by a central bank. Why would people elect to have elastic supply when a central bank could easily guarantee a fixed supply? The answer is price volatility. If a currency has fixed supply, it can't respond to fluctuations in demand without its price fluctuating. While this isn't a big problem for basic transactions, it renders Bitcoin unusable for payments over time, like salaries or loans, because the duration magnifies the negative effects of volatility. Nobody would want to take a Bitcoin loan because their monthly payment might cost their whole house after the first few years.
At Basecoin, we're creating a cryptocurrency whose supply grows and shrinks algorithmically to keep the price of each token pegged to a dollar. A crypto that can finally be used as a real currency.