avec,
I don't think anyone is seriously arguing that a Cult Membership is so good that it's worth actively seeking out one ("camping out" on the cult locations): it very clearly isn't. The issue is simply whether it's worth joining it if you're given the opportunity.
Of course the benefit of joining is extremely marginal, but so are the potential drawbacks. It *is* unlikely that the immunity to gate-open-cards will benefit you at all, but any reduction is risk is good if there isn't a downside.
While I haven't looked through the Woods and Unvisited Isle encounters, I am wholly convinced that Cult Encounters are substantially better overall than Black Cave ones.