What is the evidence to suggest that the introvert/extrovert distinction is a shallow distinction? The author here claims:


  A great leader knows that making room for both extroverts and introverts to share their thoughts could be the key to whether or not their next project succeeds or fails. 


This document purports:


  Carl Jung and the authors of the Myers–Briggs provide a different perspective and suggest that everyone has both an extraverted side and an introverted side, with one being more dominant than the other. Rather than focusing on interpersonal behavior, however, Jung defined introversion as an "attitude-type characterised by orientation in life through subjective psychic contents" (focus on one's inner psychic activity); and extraversion as "an attitude type characterised by concentration of interest on the external object" (the outside world)


The commentator writes:


  The distinction between introvert and extrovert is shallow psychology. They are sometimes used condescendingly by angsty individuals at either side of the imaginary spectrum. The truth isn't even that most people fall somewhere in between. There is no gray area, because these matters are not black and white. Their use leads to generalization and over-simplification of delicate subject matter. Introvert vs. extrovert is always a sledgehammer where a scalpel is required.


My question is: What is the evidence to suggest that the introvert/extrovert distinction is a shallow distinction?
I think it is safe to say that in psychological research on personality differences, extraversion (vs. introversion) is one of the most important personality dimensions. In fact, extraversion (along with neuroticism) has been included in "virtually all major dispositional models" (see Watson,
Clark, &amp; Harkness, 1994; Zuckerman, Kuhlman, Joireman, &amp; Teta, 1993; as cited in Watson et al. 1999). Extraversion can be reliably measured with personality questionnaires (e.g. Costa &amp; MaCrae, 1992), it is a heritable trait (e.g., Jang et al. 1996) and it meaningfully predicts other important variables such as trait positive affectivity (Watson et al. 2009) or job satisfaction (Judge et al., 2002) to pick two arbitrary examples.

Of course, neither does this mean that it is useful to categorize people into introverts and extraverts (personality research has long moved away from "type" approaches to investigate gradual differences) nor does this mean that extraversion is the only personality difference that matters (it is an important one, but there are others).

In conclusion, extraversion-introversion is a robust and widely replicated personality dimension that features in almost all models of personality.

References

Costa, P.T.,Jr. &amp; McCrae, R.R. (1992). Revised NEO Personality Inventory (NEO-PI-R) and NEO Five-Factor Inventory (NEO-FFI) manual. Odessa, FL: Psychological Assessment Resources.

Jang, K.; Livesley, W. J.; Vemon, P. A. (1996). Heritability of the Big Five Personality Dimensions and Their Facets: A Twin Study. Journal of Personality 64 (3): 577–591.

Judge, T. A., Heller, D., &amp; Mount, M. K. (2002). Five-factor model of personality and job satisfaction: A meta-analysis. Journal of Applied Psychology, 87, 530–541. doi:10.1037/0021-9010.87.3.530

Watson, D., Wiese, D., Vaidya, J., &amp; Tellegen, A. (1999). The two general
activation systems of affect: Structural findings, evolutionary consider-
ations, and psychobiological evidence. Journal of Personality and Social
Psychology, 76, 820–838.