Photography, like any arts type job, is one of those professions where a good portion of what we produce is very subjective as to what is perceived as a good product, a great product, a so-so product, etc. A license to practice photography is not needed. Because of this the person looking for a photographer can be overwhelemd as there are so many people doing it.
A membership in a professional photography association may not mean much, or it could mean a lot depending on the association and what, if any, qualifications there are to getting it. Of course a potential client might not know this. Other benefits for being a member that a client will perceive as a good thing if you belong is indemnification insurance. Where if you mess up a wedding and lose all the images regardless of how it happened the PPA through the insurance would do things like foot the bill to re-inact the wedding/reception. Or if you screw up things and it is totally your fault the client has recourse through the PPA to go after you.
Being a member will never hurt your business, but it will only help if you educate the client the reason it is a good thing you are a member. I have not had any feed back from any clients that indicated that they hired me over anyone else because i was a member of PPA. I have had people find me and hire me through my local chapter of AzPPA's website though.
The bottom line for my clients to hire me or not is one or two things or both.... they like my portfolio, and/or they got a referal from a previous happy customer.
I value my membership in my local AzPPA because I get to asociate with other photographers and get to know that they have the same problems and success as I do. I get valuable feedback from our local print competition that pushes me to constantly do better and learn what to watchout for as well as what the latest trends are in specific types of photography.