But, it does.
The purchasing power of coins did not change much, but availability does change and quite markedly.
If you will look to U.S. history, there were many speculative bubbles, boom and bust cycles which occurred despite the usage of a bimetallic standard.
War is one of the best ways of rapidly draining a country of its metallic units of exchange.
This moves government to use paper (bills of credit) in order to allow commerce to occur.