There is a long, theoretical answer to your question. I'm going to give a much abbreviated version, and some pointers to some excellent theoretical literature on the topic.
The immediate answer is that the level of the currency doesn't matter, but rather the change in the level of the currency -- or, inflation -- matters. Then the next question is, why don't economists like very high inflation? Or rather, why do economists seem to prefer relatively low inflation?
The short answer to this question (as far as a short theoretical answer can go) is that when you set up a "modern" macroeconomic model -- that is, a Dynamic Stochastic General Equilibrium (DSGE) model -- and add elements to the market portion such that prices have a sluggish or 'sticky' component, and add a central bank which tries to set inflation such that the best society-wide outcomes happen, one of the emergent results is that the monetary authority (the central bank) would like to smooth out a number of possible effects which distort how the private household behaves. A distortion here means that the private household will act in a way that is best for the HH at the micro level, but bad for the economy at the macro level (which then filters back down and is also bad for the household at the micro level). One of these distortions is that the household will want to save some money in a non-interest-bearing asset -- i.e. cash. Basically the household will want to shove some cash in a mattress. However this is not good for the economy as a whole. In this basic version of the model, the central bank can eliminate this distortion by setting inflation to zero.
This result emerges from this basic theoretical model, and is called "the Friedman rule."
Good references to this in more detail are Jordi Gali's NBER working paper, here, specifically the intro and section 5 (specifically, page 21), and Gali's longer monograph, here.
Practically, because inflation is measured with some error, central banks target some value of inflation just above zero, with the goal that "true inflation" will then be somewhere closer to zero. There are other reasons as well, but I unfortunately don't have a great reference for these points. Central banks' annual reports is actually a great place to look here, as the text is written to explain such questions in layman terms. The US Federal Reserve board, and Reserve System Banks (for example the San Francisco Fed and Saint Louis Fed) have some great resources that explain these things. The Bank of England and the Riksbank (Bank of Sweden) also have some excellent resources. (Another expert is welcome to correct this and/or add references!)