And even the people who have incentives to save bitcoins will spend them if they decide they actually need something. Also the value of their coins rising might allow them to purchase something they couldn't purchase in the past because they didn't have enough. Also it's important to remember that eventually bitcoins will have a fixed money supply and then the value will be close to fixed unless the amount of users change. More users mean the demand for bitcoins is higher and the price goes up, if it's losing users then the value goes down etc. If the userbase is fairly fixed, so is Bitcoin's price.
Of course this doesn't take into account currency speculation but that is a separate issue that affects all currencies equally. The effect speculation has on the price will decrease as well the bigger Bitcoin gets.