That way there's (a) the appearance of activity, (b) a constant stream of interesting content on the frontpage, and (c) interesting discussion in the comments. Before long, new real users would start to participate, e.g. by replying to doppleganger comments. At that point, it's inevitable that the new site would start to get traction as long as those new users keep coming back, which they should because the frontpage is interesting.
This could only work if someone had the balls to actually deploy the currently-released Arc 3.1 version of Hacker News, though, rather than rolling their own version in Rails. There's nothing inherently wrong with trying to clone HN's featureset, but it's interesting to note that not a single one of the HN knockoffs successfully cloned HN's entire featureset. Most of them were a halfway implementation.
Anyway. Just a fun idea.
EDIT: I just stumbled across a dump of HN from April 24, 2008: http://rapidshare.com/files/3129266675/ycombinator-news-2008...
It contains a snapshot of the first 172,575 items (submissions/comments) and a snapshot of the profiles of the first 6,519 users.
Have fun! Maybe someone can use the data to put together a cool visualization or something.
EDIT2: Just to be clear, this idea is firmly tongue-in-cheek.
EDIT3: Statistics time! According to that snapshot, when HN was 558 days old there were 38,693 submissions and 133,882 comments. The snapshot claims there were only 6,519 users. That would be an average of 20 comments per user and 5.9 submissions per user.