Unfortunately, as with the idea of batteries for electric cars that can be swapped out at the charging station, there is an incentive to hold onto better units, and quickly return inferior units to the pool. It's a case of Gresham's Law.
So the utopia of interchangeable shared units of anything will only be viable if those units are completely indistinguishable. If there's any kind of distribution of quality, the incentives will cause the economy of the system to devolve into hoarding of the better units. One way to work around this is to mandate regular exchange, but that really gets you into the territory of communism. Not necessarily a bad thing in theory or in some vastly different future, but a big change.