You've definitely brought up some good points, and I think I can sum it up by saying that no, a certificate does not substitute for experience, especially the kind of experience you have. Now, whether clients are willing to accept that is another question altogether, but I think most people would recognize that experience takes precedence over a certificate any day of the week, especially in translation where there is no "official" or national certification board. So as translators, I think it falls upon us to educate clients on what organizations like the ATA really are and what they represent, and how they don't necessarily govern the translation industry here in the United States.
