A poor-quality metric can still be useful.

Compared to the big 5 personality traits, MB has lower reproducibility, worse separability of traits, less correlation with other outcomes...and...maybe it doesn't matter so much for a lot of things, because it still has some reproducibility, separability of traits, correlation with other outcomes, and gosh, it sure is easy to understand intuitively, and catchy to remember.

That doesn't mean that MB is a good metric from a psychological standpoint, however.