It's good questions. Here's why this axis is the right one in my opinion:

1) As I mentioned, it has a lot of statistically significant correlations, including to all the variables of the Big 5. Example: Surveys show that % of the overall population that is each type (like INFJ) is very consistent across time and populations.

2) Beyond that, youre right, there are a lot of personality systems with pros and cons. But Myers-Briggs has by far the must supporting materials, tools, ease of use, and so on. I think its the quickest to make useful to the average person.

3) I've found it really helpful as a lens for self analysis in my own life.