do any of you hold any other professional organization
memberships? [...] Or if not, why not?
I can understand the value of a professional organisation before the advent of modern communication. There's genuine value in having a forum for discussion with your peers, keeping technically up-to-date and knowing what other people in the industry are doing.
This has been made obsolete by the rise of the internet.
I can understand the value of a professional organisation like the that has useful power to enforce professional standards, preventing employers from forcing employees into unethical behaviour. And I can understand the value - though I don't much like it - of a professional organisation constraining the supply of qualified workers to keep wages high.
Professional organisations in IT don't seem to achieve either of those things.
Some professional organisations run journals and standards committees and publish academic research and standards. I can appreciate the value of that.
Being a member of the organisation doesn't grant you access to all their publications.
Joining a professional institution would allow me to call myself a "professional engineer" which, in my country, is a protected title.
But the distinction between "engineer" and "professional engineer" is lost on most people. When I want to signal a higher social status other things are easier and more effective.
About the only thing of much value I can see from professional organisations is accrediting university courses. It's good for universities to have an incentive to provide quality teaching to balance the incentive to focus on research.
In other words, while professional bodies provide quite a list of benefits, the list of benefits I care about seems quite sparse.