Circular needles can be very confusing to a new knitter, and I feel, anyway, that it's best to learn on straight needles and get stitch identification and the back and forth down first.
The hardest thing when learning anything new is the frustration that comes with learning something new. If you try to eliminate the many things that can cause frustration to the new knitter, then people will keep knitting and not give up because they are having trouble with a fundamental understanding of the hobby.
I recently mentored a young university student who was interested in knitting. She is slowly moving forward as she has time and has tons of questions and observations (which has caused me to observe my own knitting closer) and it's been a wonderful experience in learning for both of us. She pointed out to me recently when she had a question on wrap and turn, that wasn't covered in the two knitting books she's gotten. Now, if I had not been there to talk her through it and explain things, odds are she would have given up on what she was knitting and never picked it up again, because she's new and it hasn't yet turned in to a passion for her (if it ever does).
I feel, as a somewhat experienced knitter, that it is incumbent upon me to nurture a new knitter and allow as much opportunity for that passion to grow as possible. I'm happy cheering on and teaching the building blocks for a good knitter, and let the more experienced folks here add to her knowledge as they do mine. So, for me, I try to recommend the things that are easiest to learn on before moving up, as we all much crawl before we walk.
So sure, you can easily learn on circs, people do, but, all things being equal, learning on the straights will set the foundation and once they have that down, they can easily move on to circulars and find out why most of us knit on them. Which can lead them to DPNs and Magic Looping and the wonders of tubular knitting.
Here endeth the lesson