We found this article on introversion and extraversion to be interesting. (Another article on the same topic by the same author is found here.) We agree that certain personality dimensions tend to be valued with a one-way function, (e.g. the author points out there are even self-help courses aimed at "curing" introversion and helping people discover their inner extrovert). The article is a good reminder that there are merits at both ends of the introvert-extravert spectrum. The discussion focuses largely on the subjective experience of an individual in accepting (and being accepted for) their orientation when it comes to crowds and interactions. The pictures, however, remind us of people who have achieved great public distinction despite a preference for plenty of alone time.
From the perspective of employee testing, it's important to view employment personality tests in the context of the job demands. Criteria recommends that employers tailor the value they place on certain personality traits to the job characteristics. Extraversion might be a trait better suited to working in reception rather than accounting. There are no right or wrong answers on personality tests, but in certain jobs there is evidence that people with certain traits tend to perform better than those who don't exhibit these traits. "More" is not necessarily always better when it comes to extraversion, and most employers are aware of this.
What we'd like to add to the discussion is that the construct of introvert/extravert is probably more of a continuum than two sides of a coin. Criteria has an extraversion scale as part of its personality inventory. We find that the scores of prospective employees tend to fall in a rather smooth bell curve, and not as clusters at each end of the continuum. So there's good evidence that it's an oversimplification to describe people using the "introvert" and "extravert" labels from the ends of the continuum (as the Myers-Briggs test does, among others).
Let's also not forget that people may act differently in different contexts. The list of famous introverts contained two outstanding comedians — Jerry Seinfeld and Johnny Carson. Although the definition of introvert is given as "someone who gets their energy from themselves," would anyone doubt that Seinfeld and Carson would prefer a large, packed auditorium to an empty coffee house? Some comedians might like to spend time alone; but all comedians perform best for a crowd.