Inflation, as long as it's predictable and not outrageously high, can be planned for. Crashes in value are a much tougher problem to deal with. How do you set the interest rate for a loan in a currency as volatile as Bitcoin? You don't know in a year if it'll be up 20% or down 50%. Similarly, you can't quote prices that are good for any length of time, unless you quote them in a more stable currency and do the exchange rate math each day.
Whether this matters depends on whether we're talking about a commodity to be used as an investment, or a currency to be spent. People won't spend something that's increasing in value; they'll hoard it instead. This is fine in an investment but not great if you're creating a currency you want to circulate.