Why? Well:
1) When he performs at his best, he's gonna be spectacular...when he doesn't, he's already shown himself to be a liability
2) A lot of the time, he isn't gonna be at his best, making the task of selecting him difficult
3) He's useless with the bat and in the field (a relatively minor point in general, but noteworthy in his case)
4) The amount of hype which is attached onto him (not his fault, I know), may be detrimental both to his performances and by extension, Australia's prospects
5) He gets injured quite often...I know that Mitchell Johnson's past injury record has been horrendous, but at least he has held up OK for the past one-and-a-half years...in contrast, Tait hasn't
6) The combination of Tait's unreliability and injury-prone body will lead to him drifting in and out of the side regularly, which could unsettle the side
I know this may seem like hyperbole, but I'm genuinely worried about Shaun Tait as a Test cricketer. As an ODI cricketer, he is very good, even if his economy rate is far too high, because he's less likely to be exposed with the bat and won't be worn down as quickly in the field.
For now (and maybe a little due to QLD-bias), I'm going Johnson>Tait. Also, Steyn>>>Tait in Tests and Tait>>Steyn in ODI's. Comparing 'em right now would be akin to putting fire and ice together.
But then again, this concern of mine about Tait is hardly relevant to this thread, so I'll shut up now.