I've been saying that about how much it overlaps (though not perfectly) with the Big Five for years to skeptics (who are invariably not actually psychological researchers; I haven't had the opportunity to ask a research psychologist this). During that time, nobody has been able to explain to me why the Big Five is a valuable tool for psychological research, to the extent that it's used in legitimate scientific journals, but the MBTI is utter rubbish with no basis in reality, even though there are correlations between the two.
I guess it's because the correlations are stronger. With MBTI you'll still get statistically significant correlations with other relevant measures, but perhaps when you correlate them with Big 5 dimensions instead the effect size is larger.
However, that would be because the original MBTI instrument hadn't gone through sufficient refinement to enable the test items to accurately reflect the dimension which it's testing for. I'm sure that with a well formulated equivalent inventory (i.e. one that uses the same basic dichotomies but uses better questions) you would get much better correlations, and again by including, for example, introversion and extroversion as 2 distinct and anti-correlating measures, as opposed to merely low extroversion and high extroversions, the correlations formed would be much better and more explanatory than Big 5. I mean, assuming of course that introversion and sensing, thinking, perceiving are actual independently existing cognitive patterns distinct from a merely low level of their opposing cognitive pattern... and I'm sure most people here agree that they are.
edit: one reason people tend to think of the MBTI is rubbish is due to the concept of
type, i.e. that you're one type or another type, as opposed to it being a sliding scale where you can test differently at one time or another, and have any combination of strong or weak/no preferences. Having to be either, say, an INFJ or INFP and not an INFx, is a stumbling block for people to accept the system (though, going by what reckful has said, MBTI actually does accept being in the middle of one or more dichotomies, so that argument would be invalid for invalidating MBTI).
I believe the weakest correlation with the Big Five is the T/F dichotomy. I like the way Keirsey explained it in terms of T types being reticent to engage in emotional expression vs. to the way some people used to understand it in terms of not having emotions (which usually actually just means that the people in question have extremely poor emotional awareness).
Yeah, by the correspondance between the two, that would be equating someone with a natural preference for logical reasoning with an egocentric person. The correspondence sounds off.
Feeling is largely about social reasoning, and includes the use of emotions for socially related expression, and doesn't include everything related to emotion. A Thinker would find the social use of emotion to be energetically taxing, and thus be less predisposed to it than someone who is energised by such thing. Of course, there are other factors such as male Feelers being reluctant to socially express emotion due to gender expectations.