I'd strongly recommend you join - the Belgian branch that is.
Even if only for the GA Advocacy, it's important that we have as much lobbying/promotion of GA interests as we can.
You may also find out some additional Belgian specific aspects through your local branch that aren't known elsewhere.
The other primary reason I've joined is perhaps as a bit of an "insurance policy", where I can contact them for advice or help if I have a major problem or dispute.
For those who feel that their local AOPA branch isn't acting as they would wish, then you could always get involved... But without the support (including financial) of a strong/wide membership base, such groups can't achieve what we all want.
SD