As the help says, can only send a note to a roster or a manually-entered group of students. We need to be able to send to a large collection of students based on all kinds of information about them. So, in the end, we need to send to a [advanced] list of students.
Yes- we are able to now do this through the Candidate Lists, which are great, but if we want to say, send a reminder to a mix of Candidates and Students, this can still be a hassle.