Commodity money results in artificial scarcity.
Take the case of Jack and Jane
Jack is a great gardener who could use more customers
Jane is a great web designer who has a forest of untamed weeds growing on her front lawn.
Neither Jack nor Jane have much (if any) commodity money, say, gold. And thus, neither of them engage in trade with each other though it would be beneficial to both.
Gold is scarce, and its especially scarce because the people who have it, not Jack and Jane, are trying to hold on to it, and in fact amass more.
The scarcity of gold, through people's perceived need thereof to engage in economic activity, results in an artificial scarcity of goods and services and declining economic activity. People who should trade with each other do not.
I actually do not think this is a problem with bitcoin because bitcoin can be divided as needed so even if Jack and Jane only have a Satoshi the current minimum unit, between them, the protocol can be modified and a whole economy can run on one Satoshi.
Most people people see this as a problem because they extend the analogy of commodity money too far. This is if fact one of the ways that bitcoin is disimilar and superior.