Bitcoin’s limited money supply is one of the things that people like about it: the currency cannot be debased, as money can when central bankers print more of it. But the flip side is that if the demand for bitcoins rises, for whatever reason, then the value of bitcoins will necessarily rise as well. So if you think that bitcoins are going to become more and more popular, then—again—it’s foolish to spend your bitcoins today.
It may well be the case that you expect Bitcoin price to rise, AND the expected utility gain of spending your BTC on something specific right now is worth more to you than the expected utility that appreciated Bitcoins will buy you at some point in the future.
The rational thing to do is hoard them and eventually sell them to new users. But that means there will be fewer bitcoins in circulation (and more in people’s virtual wallets), making them less useful as an actual medium of exchange and making it less likely that businesses and consumers will ever see Bitcoin as legitimate.
The infinite divisibility of Bitcoin means that ‘fewer bitcoins in circulation’, in itself, is not a problem.