Iwan Luijks has a recent post to his blog trying to dispel some of the myths and misconceptions about becoming a certified ZCE through Zend's testing. He also includes some of the benefits of having the certification to go with your name.
In the community of PHP developers there are more non-certified professionals than certified professionals. PHP developers mostly don't really see the pro's of getting certified. These developers mostly honor the fact that PHP is a quick and easy language to learn and to get started with, and professionalism is found even by building a simple dynamic web page, leaving the discussion of whether this is a good or bad thing for what it is.
He talks about the confidence it can give current (and future) employers in your skill level and let them know you're the kind that keeps on learning/improving in your career. It also lets them know that you're dedicated enough to learn what you need to pass and can put it into practice easier in the future.