What is happening is that you can think of a personality profile as a vector in very high-dimensional space. MBTI presents a basis in a 4-dimensional subspace of that. How well it works for a particular person depends on how her personality aligns with that basis.
A lot people will test near the middle on the MBTI characteristics, and say that the test does not work for them. If most of the population is near the middle, statistical studies won't show much validation for MBTI either.
Now, none of that means that the test has little value -- it actually has a lot of value, but only for people who test at relatively high scores. For example, I generally test as either ENTP or INTP -- and since I'm near the middle of E/I scale, it does not tell much either way. my N, T, and P, however, are quite extreme (I've done quite a few of these, and not once have I had a case when my N or T score was not at the end of the range, P nearly there too) -- so the descriptions and advice for these types that I've seen have been quite relevant and helpful.