Jung viewed his eight types as four varieties of extravert and four varieties of introvert, while also declaring that more people were essentially in the middle on E/I than were significantly extraverted or introverted — and characterizing those ambiverts as "the normal man." And Jung also stressed that people of the same type varied considerably in terms of the strength (or, as he often characterized it, "one-sidedness") of their preferences. Myers likewise distinguished between people with mild and strong preferences, and allowed for the possibility of middleness on all four MBTI dimensions. So the idea of middleness on one or more dimensions goes all the way back to the MBTI's theoretical roots.
The official MBTI test is designed on the
operational assumption that people have four preferences, and assigns people a (tentative) type on each dimension. But that's a very different thing from saying that the MBTI theory says that it
isn't possible for someone to be in the middle on any dimension — and in fact, the MBTI Manual specifically notes that someone with a score near the middle is someone who has essentially "split the vote" rather than offered much evidence of a preference. And the more recent "Step II" version of the MBTI has five subscales for each dimension, and it's possible to come out on the E side (for example) of some of them and the I side of the rest.
Although I'd agree that
Jung's overbroad conception of introversion included some elements of Big Five neuroticism, Myers' many corrections to Jung included pretty much leaving those behind. What E/I items on the modern MBTI do you think reflect the inclusion of neuroticism?
It's maybe worth noting that Big Five stats suggest that there's a mild
correlation between being introverted and being above-average in neuroticism, but that's not the same thing as introversion "including" neuroticism.
The latest version of the MBTI (Form M) was substantially updated in 1998, based on lots of data, studies, etc., much of it going back no farther than 1980. I believe the NEO-PI-R is still the leading Big Five test, and it's from 1993.
If you don't mind me linking to myself
again,
here's a two-part post that has quite a lot of discussion about the scientific respectability of the MBTI, and how it compares to the Big Five — and about several other issues often raised by people claiming to "debunk" the MBTI — and that cites (among other sources) a large 2003 meta-review and supplemental study that found that the MBTI was basically on a par with the Big Five in the psychometric respectability department.