While I would tend to agree with the prevailing sentiment that the test can only tell you how you think you are...
The spread of self-reported types for the forum is pretty interesting.
Discordianism - no Sensors allowed!

I have tested pretty consistently as INTP over the years, with T and P trending more towards F and J over time as I have become less of a self-consumed asshat.
I think that those people who take the test and cling to the result (and I was one of them for a while) are just uncomfortable in who they are and the box of MBTI gives them some self-definition.
Being able to hit the middle point of a few of the categories or adapt based on circumstance seems to the healthiest mode of being.
EDIT: I also think that the more you read about the methodology of the test it becomes pretty easy to figure out which questions are trying to place you on which scale and that makes repeating it a bit of a farce.