No, it is very relevant. The 16personalities site bases its theory on the Big Five OCEAN (CANOE, NEOAC; Openness, Conscientiousness, Extraversion, Agreeableness, and Neuroticism) personality trait system that is used in professional psychological research and the generally accepted personality trait structure under existing consensus of psychological science, but adapted with theory and concepts from the Jung/Myers-Briggs systems as well. This OCEAN model was developed by statistical analysis (factor analyses) of personality reports and descriptions to isolate the number of independent trait axes involved. Four of the five dimensions (E, O, A, and C) correspond more-or-less to the four Myers-Briggs dimensions (although they don't always line up, but it could also be by the different framing of the traits in how the tests for each are usually constructed) - but the fifth one, N or Neuroticism, does not (although a factor analysis on one copy of an "official" (CPP, or whatever the company is called, too lazy 2 google right now) revealed that hidden "unused" items on it did actually measure the Neuroticism dimension, but that was a long time ago, I wonder if/how those items are integrated to the more recent ones (I am not made of $ and so can't shell out on taking and seeing what the "official" MBTI tests are like and even then, if I did I'd only see the latest)). The A/T on 16personalities is the addition of that extra Neuroticism dimension that is missing from the standard MBTI. Neuroticism is a well-documented trait axis that is not just a function of the other four, so there's no doubt it exists :)