I have had occasion when I wished I had a tape recorder, and others when I've taped on particularly difficult topics, and it was helpful. Then there are the occasions when everything goes right and you get a gem of a lecture, one way or the other, and it is an important record. I was in a grad school philosophy class several years ago, and we'd been reading a variety of texts that took us through a series of world views, and then one evening the professor decided to tie it all together with a lecture instead of our usual class discussion. My handwriting is usually pretty sloppy, but I was more coordinated than usual and I got most of it down in clear notes. I also had the good sense to transcribe it as soon as I got home, so I could include additional material I remembered but had only made obscure references to (like your note above). In the long run I was actually able to cite that lecture in papers, because of making those notes. On tape it would have been a classic.
This professor is one of those much-published folks who years from now will likely be the subject of biographical works. A lecture like this would then become a perfect example of the quality of his lectures, as recorded by a student.