For all of the "science" that goes into building the models for these tests (not just MBTI), any that rely completely on self-evaluation (especially multiple choice questions), should be taken with a huge grain of salt. They're fine for entertainment, but for anything practical they should be regarded as one step above astrology or haruspicy.
Just to provide an example of the worst case, I've seen companies hold personality test workshops, only to have that bind their thinking and give them reason to quash dissent ("Of course... that's what a XXXX would say" or "I really need a YYYY on this project"). Too many times these things turn into a substitute for actually getting to know your employees' strengths and weaknesses.
I don't want to be seen as overreacting to a simple harmless poll, but I do want to make sure people aren't giving too much credence to their (or others') results.