I wouldn't technically consider Bitcoin as a global currency, as it still isn't very widely known. Countries tend to like to have total control of their coin, and since Bitcoin is a P2P currency, their only kind of control is having a huge share of the 21 million coins. I really don't think it will become a global currency, as it is quite an unstable little thing.
If someone/something want to control BTC by buying a huge share of BTCs, it will cause that there will be a large demand for the remaining coins and maybe it will detour the system from the normal way of working. If a huge part of coins sits somewhere under control' it can happen that people won't have enough coins to do the everyday transactions, it can make other similar cryptocurrencies to raise possibly.