Why do so many introverts have a hard time deciding upon their type in MBTI? So many authority figures in the MBTI scene claim what you claim: that E/I isn't about being being social/withdrawn, only to contradict themselves in their questionnaires and their type descriptions (again, perhaps not official MBTI, but most of the derivates do).
Yeah, and come to think of it, I've always come out ENFP or ENTP when I take the official test, even though I think of myself as an "I."

And to bring things into balance: Socionics sucks as well! Not enough info available in English, and if there is, often way to theoretical and abstract and typical "NT"
I think the biggest weakness in Socionics is lack of agreement on some fundamental issues and definitions. Even different sites' VI pictures look so different.

The one area where MBTI is the strongest is that there's a lot of statistical research that uses one standardized instrument; and a lot of statistical research went into creating the MBTI as well. As a result, there's a lot of hard data to look at....and, at least initially, practitioners stuck more or less to what was supported by hard data in drawing conclusions....but that may have changed as the MBTI got more popular.