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.
In real life, if someone takes a brochure of a course because they're interested but never show up afterwards, you don't count those people against your completion rate (you might count them when evaluating your marketing funnel, though). Similarly, for a MOOC it makes no sense to treat the number of "signups" as anything more than that.
If you want a reasonable completion rate, you divide the number of graduates against the number of participants that (a) showed up to the course - actually listened to more than 20 minutes of lectures and (b) actually wanted to complete the whole course, as opposed to wanting only a single topic/part of the course.