Well, as a former physics TA I suppose I could take umbrage at a couple of the characterizations, but I won't. Suffice to say that at least I put some time into my teaching efforts, and my student's comments after the semester bore that out. Also bear in mind that most TA's (in my experience) are also taking a full course load of their own. They are also usually in the first two years of their graduate study, in many ways the most intense and stressful since that's where the "weeding out" takes place. So the attitude some of you have encountered is understandable, if not acceptable.
As to postdocs, the best analogy I can think of is the internship and residency an MD has to go through. Intern's are doctors (they've finished med school) but can't practice medicine without this additional training. Postdocs aren't as required, but they are traditional as a chance for a new Ph.D. to do some research without the pressures of writing a dissertation or of being a full faculty member. It's unusual to find a postdoc teaching.
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind." - William Thompson, 1st Baron Lord Kelvin
"If it was so, it might be, and if it were so, it would be, but as it isn't, it ain't. That's logic!" - Tweedledee
This isn't right. This isn't even wrong. - Wolfgang Pauli