In fact, the secondary functions money serves invariably end up hurting the two essential ones.
Some examples follow.
At first sight, it really is convenient to have money also play the role of store of value. Therefore, at the first signs of a recession, anybody who has money will logically save more and consume less, thereby exacerbating the recession for everybody else. In boom years, consumer optimism prevails, and people will tend to simultaneously dip into their savings to buy big-ticket items such as cars and houses, thereby pushing the boom into an inflationary period. While other factors also play a role in the creation of business cycles, it has been proven many times that consumer confidence significantly exacerbates the problem. Providing incentives to ensure that the medium of exchange does not also incorporate the store of value function would therefore automatically dampen this boom-bust tendency of the current system.