Philosophers active in the profession -- if they are employed full-time in a U.S. university, especially a high-profile university with a Ph.D. program in philosophy -- have, I think, some obligation to the philosophical community for the support and nurturance of the profession. One may fulfill this obligation in part by doing things like refereeing essays, chairing sessions, serving on committees, and paying dues to professional organizations, including minimally the American Philosophical Association. The APA in particular forms committees and publishes newsletters and proceedings pertinent to issues in the profession, organizes three annual conferences, supports and publicizes awards, and provides an admirably well-organized and equitable structure for advertising positions in philosophy (including keeping a list of censured institutions).
Some active U.S. philosophers, I'm sure, have good reasons not to belong. But I'm inclined to think that at least for leading members of the profession, it's a small, defeasible wrong not to belong -- a bit of freeloading or a small lapse of generosity, perhaps. (Of course, we all have our lapses in one arena or another!)
Even if there is no obligation -- not even a weak and defeasible one -- to be a member, it still seems reasonable to suppose that ordinarily, and all else being equal, supporting the organization financially by one's dues and membership, is a good thing, if you are an active, prominent, full-time member of the profession.
Or are decisions about membership simply decisions of prudence, so that if you personally derive no benefits from being a member (you can do without the newsletters and proceedings, you are willing to pay the slightly higher registration fees when you go to meetings, you don't care to serve on any committees) paying membership fees is only foolish, like buying a pair of shoes you'll never use?
(Let's say these reflections are a propos of the following statement in the On-Line Philosophy Conference introduction:
Finally, we are very pleased to announce that Professors McMahan and Sosa have generously offered to donate their keynote honorariums to charity. This year the charities selected by the OPC keynote speakers are Amnesty International, Oxfam, and The American Philosophical Association. Please follow their generous lead and donate what you can. If nothing else, treat it as an inexpensive conference registration fee! We have provided links in the sidebar to this year's official charities. We hope that with your assistance we can start a charitable tradition here at the OPC, and we thank both Professors McMahan and Sosa for laying the groundwork!)