I've recently thought again about potential membership of professional bodies. I used to be a student member of the ACM - and, despite a steep discount, I felt there was little value there... so dropped it as soon as my discount eligibility changed.
The idea that a professional body should prosper by restricting access to content might work in academia, but it does not represent a compelling proposition to me.
I would consider joining a professional body if it were:
1. Relevant to professionals who work with software - neither pandering to esoteric academic nor lowest common denominator content.
2. Needs to involve (fairly local) physical gatherings "Conferences" - at which I will meet relevant people and discover interesting things beyond what can be found on the web.
3. Needs to be recognised widely as conferring an active interest - to bolster academic credentials and professional engagements.
Not only does the ACM fail to meet even one of these criteria... I can't find any other organisation that does much better. In fact, I came to wonder if membership was actually counter-productive... does it suggest someone who is not sufficiently confident in their other credentials... someone who hopes to buy recognition.