It did happen. I was there (Laurentian University in Sudbury Ontario), teaching in the Department of Physics and Astronomy and my former colleague, N.I. Robb now retired - and an extremely well liked prof btw-, did just what I described.
People get really annoyed when someone undeserving gets favorable treatment. So, stronger students will not take the chance to let weaker students copy their assignments. Penalize both equally, and a group mentality makes it more likely that they will take a chance again... at least, that's how I understand it worked out.
But, the students that do care about the course are dissuaded from plagiarizing their work. If my experience in uni is any telling, people who get caught copying/plagiarizing will do it again. If you cut the easy access of the more hard-working students, at the very least they'll have to do some research for those answers.
The nice thing about the zero to the stronger student without consequence to the weak student is that they didn't go down together, which tends to make the affected more likely to go confront the issue.
Sure, what I'm saying is not based on any hard data except my years at uni and the fact that a physics professor in my campus did the exact same thing.
wouldn't have made for as good or as memorable an example. the guy who let the other guy copy his work now has to feel bad both about getting a zero, and about seeing the other guy unfairly get 100%.
