My feeling is that GA is too small a community and too easily mis-represented and mis-understood in the wider public community to allow us the luxury of not joining and supporting at least one of the representative organisations. Such organisations have the connections where it matters, most noticeably the CAA and the government, to stand our corner. ISTM that regardless of whether you feel that they have been effective in doing so, without them we would not even have those voices and I dread to think what state GA would be in as a result.
AOPA, as well as PFA and BGA, amongst others, have been instrumental in setting up, running, and supporting the GAAC (www.gaac.co.uk) which should, IMHO, be regarded as a major success story in terms of representing the interests of GA in the UK. As far as I am concerned this alone justifies the annual fee I pay as a card carrying member of AOPA.
Amongst other things the GAAC watches over local council policy and works to get GA interests represented in the planning policies that can see an airfield thrive or disappear - over 300 representations made since 1995. In that time the quantity of structure plans which have a positive reference to GA has risen from 20% to 63%. They also scored a significant victory recently in having PPG13 (which forms central govt guidance to local authorities on planning policy) re-worded more strongly in favour of GA.
If you really don't feel that you can support AOPA, PFA, or whoever by joining, then another option might be to donate to the GAAC the membership fee you think would be wasted elsewhere each year.
I would also make the plea that it is not enough simply to pay a membership each year. The value of individual expressions of support on specific issues should not be underestimated. See
www.airfields.org.uk for info relating specifically to airfield issues.