Like it or not, there are things that can be measured, and when you do, you get different answers. Introversion/Extroversion can be measured, and people get different scores.
Some people get different scores at different times or in different contexts, but that doesn't mean the measurements are meaningless or valueless.
Understanding these issues can be of tremendous value, and can help to ensure that unusual personality types are not needlessly "treated" or harrassed. Instead, others can be encouraged to realise that different people have different needs.
Actually, most scales of introversion and extroversion (some people spell it extraversion) are quite poorly validated, and that especially goes for the Myers-Briggs questionnaires.
I accept the idea that some people on particular occasions enjoy the company of other people more than others, but it's considerably less clear that this is even a stable trait, rather than a state, on the individual level, and many of the inferences drawn from classifying a person as "introverted" or "extroverted" are highly debatable.
Some people get different scores at different times or in different contexts, but that doesn't mean the measurements are meaningless or valueless.
Understanding these issues can be of tremendous value, and can help to ensure that unusual personality types are not needlessly "treated" or harrassed. Instead, others can be encouraged to realise that different people have different needs.