How does DRK solve any problem for merchants and clients?  I have bought items from reputable merchants using Bitcoin. I didn't notice any problems.  In fact, every purchase I have made from overstock.com was flawless and immediate.  There isn't any problem that needs to be solved.    
 and as long as you pay taxes when you convert to fiat they
DRK does indeed solve BTC's traceability when it comes to money laundering and other nefarious purposes.  

The beauty of BTC not being anonymous is that it will be adopted by the mainstream and used for actual financial transaction.  I still have not heard where you will be able to spend DRK or even any reputable vendor embracing DRK as a payment method.  It likely won't happen because businesses are not going to put themselves in a position where they could potentially run into trouble with FinCEN.  In case you don't know, FinCEN "Financial Crimes Enforcement Network http://www.fincen.gov/ " mission is:
 
"FinCEN’s mission is to safeguard the financial system from illicit use and combat money laundering and promote national security through the collection, analysis, and dissemination of financial intelligence and strategic use of financial authorities."

I would hate to have my fortune tied up in an anonymous cryptocurrency that could be shut down at anytime by the United States Government using the Patriot Act and National Security as justification.  They find a Darkcoin wallet on a seized terrorist laptop and it's lights out for Darkcoin.  Any company hosting masternodes in the United States will comply with any order to shut down Darkcoin network traffic.  European governments would follow suit, most South American countries will do whatever the US tells them to do and the Chinese don't want any crytptocurrency.

What many fail to see is that the main selling point of Darkcoin could actually cause it's demise.  Do you actually think that the United States government is just going to allow people to send whatever they want to whoever they want?  Potentially funding terrorists or laundering money to avoid paying taxes?  Not gonna happen.  Congress would find bipartisan support to enact laws to prevent just that from happening.