Certification is important primarily because it gives a client some assurance about the abilities and competencies of the examiner. That’s very important when choosing a forensic document examiner.
By ‘client’ I mean both the individual hiring the FDE, usually a lawyer or private citizen, and the Court that will eventually rule on the matter.
Does the lack of certification mean an examiner is not competent? Not necessarily. Certification is voluntary and not all examiners feel it is needed or even important. However, the lack of professional certification means that you, the client, will have to determine whether or not the examiner is competent and qualified using some other means.
This topic discussed at some length in a post you can find here.