The problem with any personality indicator is how rapidly results can change. I fluctuate between INTP and INTJ depending on time of year or who I had to deal with the day prior - and depending on specific social context I am very, very much E.
As a general indicator, sure - it confirms our own self-conceptualization, and can give general guidelines to others as to how we'll respond to stimulus. But the stress is on /general/. Its predictive ability leaves much to be desired.
Grouping by the letters slightly hides that each is a continuum, from P to J, from I to E, etc. Lots of people also fall around the middle of some of these.
As a general indicator, sure - it confirms our own self-conceptualization, and can give general guidelines to others as to how we'll respond to stimulus. But the stress is on /general/. Its predictive ability leaves much to be desired.