> I have signed up for updated versions of courses I had finished earlier simply because I wanted to show three minutes of lesson 4.2 to someone, and I've told my students to "sign up" for course X because its materials at week 4 explain a particular concept very well, and I'd recommend them to watch that - but the materials aren't available if you don't sign up.

Yes, I really wish Coursera et al would have "lecture archives" which didn't require login or, if they did, didn't require joining a class. This would help boost their completion rates, so it would be mutually beneficial. I suppose a few of the partner universities might like having restrictions on their content, though.