The case is much clearer outside of the US, where the drawbacks of centralized currencies are more visible. For example, Argentina's rate of inflation was 30% in January alone.
Say Goldman Sachs decides to enter BTC. What % of BTC in circulation do you think the biggest investment bank can buy? Let's settle for a 5%.
Then Agentina decides to dump the USD and go with BTC. Suddenly GS wants to make the 5% share jump to 6% without spending money, because that's what they do! The broker starts dumping (selling) massive amounts of BTC to the market. Everyone goes crazy and starts selling too, because the price is diving. At the same time and without notice GS starts buying, while everyone else is selling/dumping. Now the Argentian government is way poorer, because they used BTC to buy services when BTC was trading for 450 USD but now they have to sell, and because of GS, BTC is trading to 150 USD.
Having to rely in a foreign currency as the USD/EUR, essentially gives you the ability to get many more short-term loans, because it adds value to your 'products' but gives away control. You can't control your currency anymore so you can't make policy decisions based on your needs. See Greece for example. If Greece didn't had the EUR, it could never have gone in that debt. Additionally if it has the Drachma (old GR currency) it would have been able to devalue and boost exports. Now Greece depends on Brussels and the only thing it can do is cut expenses which leads in vicious cycle of poverty.
What you're describing, though is even worst. Having to rely on a currency that is controlled by whom has the biggest %, in a totally anonymous scope, is madness in the purest form and a recipe for disaster. Worst than anything you've ever seen because it will be instant and without notice.
Imagine what would have happened to the UK in 1992 (Black Wednesday), is the Bank of England wasn't able to exit the ERM. GBP would have ultimately fail in one week time.
