There are some plusses to Mensa, mostly with respect to their size. The more exclusive societies are, IMO, more attractive to those who meet the entrance requirements, but of those who do, most also belong to Mensa.
Some years ago, there was an interview with Dr. Ware in the Mensa Bulletin. He explained how the entrance requirement happened. When the group was formed, they used a self-administered test, that was distributed by mail. The intent was to screen at the top 1%. After some time, they came to realize that the acceptable score that had been used was actually screening at about the 98th percentile. Instead of revising the testing to establish the intended 99th percentile, they just changed the entrance requirement.