[TheCleric]

A few reasons in addition to what others have added.

1) I have yet to see an instance where they have been proven to be repeatable (as in the same person scores as the same type consistently).

2) All of it assumes that the person taking the test is telling you what they really are, and not what they think you want to hear. Some tests do try to ask the same question in multiple ways to trip someone up, but anyone who is paying attention can get past that. So now you're not testing their personality, you're testing their projection of a personality.

[drewcoo]

They're also self-fulfilling prophecy. The diagnosis becomes the disease.

A person who's told their entire life that they're a Leo will start to recognize Leo-like behaviors and reinforce those.

A person who's told their entire life that they're introverted/extroverted et al. will do the same.