What happens in that people make those agreements with the inflation/deflation rates already in mind, and Bitcoin actually has an upper hand since its rate of growth follows a stable pattern, unlike currencies that can be printed at whim.
look at any of the US dollar indexes, which examine the exchange rate of the dollar against other currencies to shake the belief that the dollar is stable: https://www.google.com/finance?chdnp=1&chdd=1&chds=1...
Keep in mind that commodity based currencies (like gold) have existed for thousands of years. Bitcoin acts more like a commodity than these because there's a specific upper limit on the maximum quantity and you can't discover new bitcoins (at least after they are all mined.
But yes, this could be considered a good thing (no one can control it) or a bad thing (no one can control it).