I have taken three and I have never applied for Mensa. Once in school when our biology class was curious about the topic; once online because somebody challenged me to it; once because it was in the TV magazine and I was bored, and I wanted to know if one really gets better at it. (I think I did)
I suspect you wouldn't have gotten demonstrably/statistically much better with repeated takings. Maybe if it was the same test multiple times, but even then I'm not sure.
A few years ago I had 3 IQ tests administered to me over several weeks. There wasn't any major difference between any of the scoring/ranking, despite the fact that I was taking multiple. I never got much 'better'.
I think it depends on many factors: Will I be told which answers were wrong and what would have been the right ones? I had similar problems with ambiguous questions as the OP and this might help me to understand what the test writers understand as "canonical" or "simplest" answer. Also, I am rather excited when I take a new type of test, so I guess my biggest jump would have been from the 1st to the 2nd test.
