The covariance of the Big 5 and MBTI is high. This means that if you give both inventories to the same population you can do a factor rotation of one onto the other with much of the variance being preserved. This has been demonstrated multiple times in the academic literature. I have never read an article against the MBTI that mentioned this, and that's because the people who write these articles do not understand statistics. The MBTI is approximately as valid as the Big 5.
That said, I don't think it's quite true that "The MBTI is approximately as valid as the Big 5". It seems to me that the Big 5 is a strict improvement on the MBTI. First, the discrete nature of the MBTI incorrectly suggests that the distributions are bimodal, when I don't think anyone thinks that's true. Second, I'm willing to bet that even if we just concentrate on the 4 factors of the Big 5 that correlate with the (non-discrete version of the) MBTI factors, we'd find significantly higher validity for the former, if only because there have been many more serious scientists studying and refining it over many years.
They both have strengths and weaknesses. Strengths of the MBTI include the text descriptions, which are very valuable, and that it focuses on positive psychology. Of course, if you rotate the MBTI onto the Big 5, you see that it does in fact measure neuroticism, although not that strongly.
What you have to understand is that statistically the models are very similar (you can compress them both into one unified model that does what both of them do quite well). However, the ways the models are constructed makes them useful for different things. The Big 5 is primarily useful for academics, and the MBTI is primarily useful for the rest of us.
If you are a logical positivist and scientific realist you'll never be able to grok this. As a utilitarian I understand that science is the process of making something that does something you want done.
I think I grok that some simplifications are more useful and teachable than others, and that it's possible to accomplish useful things by simplifying (and also by misleading). But I don't think you have to be a hardcore logical positivist to think these are distinct notions from "validity". My impression is that you have psychological or statistical training, so when you used the word in your original comment I assume you know what it meant.
Interesting. Looks like extroversion is the best correlated (.74) which I suppose makes sense. But nearly as strong (.72) is intuition/sensing to openness to experience, which I would not have predicted. Conscientiousness to judging/perceiving is only .49 and agreeableness to thinking/feeling is only .44. MBTI has no good correlate for neuroticism.
I'm no statistician, but "is approximately as valid" looks to be overselling the situation.
0.5 correlation means it is maybe perhaps correlated given Emax view of the statistics. For normal variates you should aim for R=0.95 at least. Both MBTI and Big Five are approximately normal variates.
Also correlation is a linear operator which is not particularly sensitive to anything nonlinear.
Use a good statistical test instead.
