This weakness is the exchangers who convert cash to e-currency and e-currency back to cash again. “Authorities” target these exchangers and the system fails (see the story of e-gold.com). If the “authorities” do not shut them down they then regulate them and privacy is eliminated (see goldmoney.com).
Bitcoin must overcome having to be converted through an exchanger.
money will follow the path of least resistance. if it is easier for me transact using bitcoins than it is for me to use a payment network (visa / mc / mpay / paypal) or the banking system (ach / sepa / bank wire) for purchasing goods, services and investments then i will use bitcoins instead.
the weakness exists in the monetary systems that self-impose more resistance than that found when transacting using bitcoins.