I agree, if you want to just buy and sell through a (supposedly) untraceable market like Silk Road, then bitcoin seems to work great. However at some point most people need to take those bitcoins and turn them into cash to interact with the rest of the world. Also, you need to obtain bitcoins in the first place and many of us have no interest in mining or selling on Silk Road to obtain bitcoins anonymously.
One solution I can think of is Silk Road sellers actually mailing you cash for you bitcoins and vice versa (cash can be sent insured through US mail up to $25,000). However, that would perhaps incur as much risk and commissions as buying from some sketchy individual in person.