This whole discussion seems to be missing a large part of MBTI's value. MBTI is not just about knowing yourself, but also understanding how other people in a group might be feeling/reacting, regardless of whether or not that reaction is indicative of their personality all of the time.

I've found that an understanding of MBTI has helped me immensely when facilitating or leading groups. Knowing that there will be some people that are most creative when brainstorming on their own (often introverts) and some that are most creative when working in groups (often extraverts) means that I give time for both in any session I'm leading. If I'm running a meeting that is going to go over time, but is producing good results, I'll ask everyone if they are ok with going over time and will set a new time limit. Before I knew about MBTI and realized that people with a J preference (unlike myself) often disengage from any process that doesn't appear to have a defined ending, I was losing out on the contributions from all Js in any meetings gone over time.

Those are just a few examples, but hopefully they show that MBTI isn't just about 'knowing your type'. People will act with preferences on either side (I lean a little bit to the I side, but often play the role of an E depending on the group I am in) but understanding how all of these preferences play out in groups gives you a much better chance of getting the best contribution from everybody on the teams you work with.