I'm not sure. With Drupal's release cycle you'd have to re-take the exam every
two years at the very least. I'm not sure if people, especially self-employed
developers, are willing to pay up whatever it costs take just to get the
certificate to "prove" clients they are proficient with Drupal.
To support this point I refer to the Drupal Services page :
"Note: We only list individuals and companies that have contributed to Drupal.
Even though a "contribution" is somewhat subjective and hard to define, it is
fairly easy to understand once you see it. See this discussion  on the
Development list for more definitions of contribution."
Your contributions prove you are good at Drupal, not some piece of paper.