Oh, I would certainly join a union had I not had the capability of negotiating a better deal for myself than the pretty general union-negotiated variety. I'd choke on my own hypocrisy a bit, but my own financial interests would likely win me over... To a degree. I'll come back to that unless I forget. But then, I DO have the abovementioned negotiating capacity, and I think people should develop that rather than rely on someone else to work in their best interest. Granted, it's not as simple as I put it in a few lazily typed lines, but there it is.
What it comes down to, is that I support people getting what they deserve based on the current standard of deserving that best applies to them. Organised workers can often end up getting more or less than they deserve since unions don't know their personal capacity to generate profit, directly or otherwise, for their employer. Me, I won't settle for less, nor would I take more than my due. Not in an employment situation. My work is worth what it's worth, and it's my job to convince my higher-ups of that fact.
That out of the way, I was not aiming at political organisations specifically. I generally dislike "interest groups" for meddling outside their defined scope. To put it this way, if you are a railroad commuters' club, then don't fucking meddle in public education policy. Go ride your trains and leave that stuff to them other assholes.
Now, some groups are better than others, of course, but it seems like you have to buy a fuckload of political agenda, not necessarilly your own, along with pretty much every organised stance you take. Indirectly supporting such agendas by virtue of an active membership is still support, especially in paid membership organisations or those getting government funds calculated based on their membership lists.