If you join because you feel you have to then that's up to the individual, most jobs will still ask for iosh membership in the belief that this and this alone makes you competent, some ask because they don't know what it is and some ask just because they have seen it on other job adverts.

I joined because I thought there were benefits, realised not always, but hey at least we can continue to argue about the competency of TechIOSH, DipIOSH and CMIOSH until our hearts content.