And you can say that Hacker News will stay pure or whatever but this is exactly how things began with other sites and then slowly but surely things became less and less relevant. I do think that it will take quite some time before Hacker News gets that way but I'd rather be strict now and nip it in the bud before regretting it later.
With the analogy of tags I would say that if you had the two most basic tags: "hacker-related" (programming, science, tech and related fields) or "non-hacker-related" (ie. sleep habits) Then I would only have hacker-related tagged material here.
Now I realize that in the case of Digg they did purposely branch out to expand their community. And I doubt pg would do the same. Thus the speed at which Hacker News would descend in quality would be much slower, but in my mind it's not a sense of speed it's a sense of direction. And right now Hacker News is descending from the direction and the kind quality we had.