I'd argue it's orthogonal. >And of course, junk traffic breeds more junk traffic. Even if the first generation of a shady subreddit might make this up with qua

I'd argue it's orthogonal. >And of course, junk traffic breeds more junk traffic. Even if the first generation of a shady subreddit might make this up with quality in other parts of your site, it will invariably attract generation 2 and later which will create a mini-eternal September. You can argue both ways, the quality increase will attract more quality users.