If it weren't for the fact that the U.S. charges capital gains tax on every little purchase, I'd happily spend my cryptocurrency and just replenish it as I go. Trading between currencies makes this a non-issue.
Also, if you're really convinced a cryptocurrency will go up, your same argument suggests you should buy the cryptocurrency rather than spending dollars on anything else. If you're not doing that, it indicates you're not totally certain of the future after all, so you can just as well spend your cryptocurrency.
There's also the macroeconomic effect of deflation to consider, but that's only an issue if there's only one currency. What we're actually getting is the competing private currencies advocated by Hayek. It doesn't matter so much if one currency deflates as long as the total money supply is sufficient.
Also, if you're really convinced a cryptocurrency will go up, your same argument suggests you should buy the cryptocurrency rather than spending dollars on anything else. If you're not doing that, it indicates you're not totally certain of the future after all, so you can just as well spend your cryptocurrency.
There's also the macroeconomic effect of deflation to consider, but that's only an issue if there's only one currency. What we're actually getting is the competing private currencies advocated by Hayek. It doesn't matter so much if one currency deflates as long as the total money supply is sufficient.