Myers-Briggs, IQ tests, etc..
I think they tend to provide valid descriptions, which can be useful for understanding people in a broad-brush type way.
I don't think they are necessarily complete - they leave off important information. Neither do I think the different categories they come up with are necessarily independent - one might well have a strong link to another. Nor do I think they are prescriptive. But they can be useful as a descriptive tool.
I guess the real dangers are either when we take them too seriously and feel restrained by them, or when we think that someone's score on an IQ test, for example, affects their value. Real value comes only from God, and has nothing to do with IQ, M-B personality type, or anything like that.