I first took to the MBTI system because it made it possible for me to explain myself with greater precision to my father. Well actually once he'd tested me he kinda found that things fell into place and he understood a little better, which was nice if a little overdue.
After that point I've seen him progress from a gruff and abrasive ENTJ into someone who actually negotiates people with skill and finesse. Gone are the points when people are declared as stupid, unless he truely does think they're being stupid. As for myself after resuming my interest in the MBTI, shortly after joining INTPc, I've noted that I handle things with a little more confidence. I can now dissassemble things with new tools and pose new models for consideration. It makes predictive analysis a whole lot more fun and increasingly more accurate. Not because I use the system to predict so much as I now have better, more accurately defined labels under which to group certain responses. This gives me a better background pattern to work with.
Basically looking into MBTI re wrote my intuitive matrix so that now I have clearer definitions to compare things to. I've stopped grouping certain traits together having now actually looked at people not as certainties but as accumulations of preferences.
Now, you really should submit that to BPTeditor@cox.net for the upcoming Bulletin of Psychological Type's issue on the practical impact of type.