I taught two people to purl yesterday. Why is this a big deal?
One of my students was taught the knit stitch at another shop and has been contently making her scarves, row after row of the same stitch. Now her son wants a "Harry Potter" scarf but he doesn't want the bumpy look of garter stitch. She didn't want to work in the round so had to bite the bullet and learn to purl. (See, Hannah, it can be done.)
While she was practicing her new found skills another lady arrived who wanted to learn to knit. She had already tried to teach herself but the beginner's book she had didn't really deal with the how to knit part. So I cast on for her and showed her how to do the knit stitch. After an hour or so I showed her how to purl. And then how to cast on and off. Student number one was so impressed. "You've already learned the purl stitch!"
I almost always teach the knit and purl stitch in the first session, time permitting. I don't see any benefit in postponing the purl stitch. I think it's much easier to do than the knit stitch anyway. And it opens up so many more options than just knowing the knit stitch. There are some people that I can't do this with but most of the time it's no problem at all.
Anyway, that's my Knitting 101.