from what i know (which could be all relative since every single school here seems to have its own rules and regulations), being a union member here doesn't matter much as unions are illegal here, and there's pretty much no one to defend your rights legally if you have a spat with your employer.
that being said, if (God forbid, it's not something anyone here would look forward to) you have to sue your school here, you should get a lawyer who knows the local labour law and (ideally) speaks Arabic, as your beautiful english language is the lingua franca here, not the second language, hence all legal proceedings are dealt with and conducted in arabic in court by an emirati judge.
i used to be a member of a number of professional teaching and research associations back in europe, and frankly speaking, unless i opted to pay for the membership regularly and keep an active profile in their activities, it didn't make sense to keep being a member.
now, if you think you're going to be here for a year or two, and then go back and enjoy the benefits of belonging to a union, maybe you should make an effort and keep the membership(s) if you currently have one/some. if you plan to kiss your beautiful country good bye for good or for a longer time, then...
why don't you speak with the union reps you have there and see if the benefits will apply if you are abroad for a number of years? given that you keep paying to the union of course.