Unlike real currencies it looses value when the price goes down. If $ goes down 3% tomorrow there will be no difference in the price you pay for stuff but if BTC goes down the price for stuff goes up. That is because it is 100% linked to dollar and does not have a value on its own.
Many nations want their currency to get lower exchange rates because it is easier to sell their goods to other nations. Only nations who import a lot of stuff want a high exchange rate. Bitcoin has realy nothing to do with trading and it has its whole value based on current exchange rate.
You have a good point about this, but the value of fiat is controlled by government, so if the government change the price for stuff because its price changed, then economy situation of the country would be ruined by that. But with bitcoin, it doesnt matter because it is totally controlled by us, even its price for stuffs always changing time by time.
By the way, since the bitcoin price is being decreased, people couldnt hold their bitcoin any longer and began to sell it, then its price will be more decreased. Seems reaching US$500 will be a delusional thing for this year.