People are far more sophisticated than any basic yes/no test could ever hope to encompass.
I hate the MBTI and it's cousin True Colours - where they simplified the MBTI's 16 types down to four. People have found it useful in understanding themselves, but where it's dangerous is when they use it as an excuse for not communicating better with other people. By the way, the MBTI was not intended to be used as an excuse, but as a tool to help types communicate with one another.
I still hate it though. Malcolm Gladwell published an article in The New Yorker in 2005 which is available here. This article goes into great detail about the failings of the Myers-Briggs test and is a good read.
The MBTI has led to some positive things. At least imo.