And it's free.
The page to the .pdfs is not found,
so here are links to the html versions:
SINGER-LOOMIS TDI: THE NEXT GENERATION OF PSYCHOLOGICAL TYPE INSTRUMENT S. Dugan Haskayne School of Business
Understanding Personality Functioning Without Forced Choice:
I have a feeling people still play with MBTI mostly because the material is freely accessible (as are the tests) for the most part. Better systems have probably been made (even from the same source), but they all cost money and can't be discussed or theorized about quite as openly.