So why would someone join a club which is set up to say that they are the brightest in the nation, if its not to look down on others that they consider less intelligent? (Ignoring of course insecurity of their own intellects)
I already explained why I joined :
* ) It can be a plus for your resume.
Nah, that is what a first class degree is for.
* ) It can make it easier to find people interested in intellectual activities like philosophical debate or abstract board games.
There are better ways of doing the above as opposed joining Mensa, e.g academia, political parties, think tanks and other such clubs.
* ) It has a special interest group focussed at researching and debating common side-effects of high intelligence like Asperger's.
Most universities research it anyway, with access to laboratories, the whole academic community and so on.