Do it if it adds to your CV, otherwise there's no point.
I used to be in the IMechE when I was a design engineer until one day walking down Birdcage Walk I passed the headquarters at about 3pm and staggering up the steps were two red faced fat blokes returning from a good lunch. I thought if this is what my fees go on then you can stuff it. Being a member or not has made absolutely no difference to my career.