It's basically a mathematical guarantee. Reddit shows 200 comments by default, so if there are 20k comments, then you are roughly getting the top 1% of comments. If there are fewer than 200 comments, you are getting ALL the comments, basically unfiltered. I've also noticed that the best users make a greater effort on comments that may be read by a large number of users (granted, the average user probably does not)
Voting quality clearly goes up with scale. On a small subreddit, you can make a brilliant comment, and only one person votes on it and you are at score 0. On default subs, the voting tends to be more accurate (though with a populist/groupthink bias)
I think some are confusing comment quality with comment value. A really funny, but silly joke would be a high quality, low value comment. The default subs tend to be lowest common denominator subjects, like "cute" stuff and politics -- low value. Whereas niche subreddits are highly focused on your particular passions and interests. Niche subreddits have greater value to users, which is why they are also more valuable for advertising and monetization purposes.
At sitewide scale, the quality of existing niche subreddits would improve dramatically, and smaller niches that do not exist today as subreddits would gain sufficient scale to support independent subreddits. I think that these niche subreddits could support almost search keyword-like targeting (albeit with lower CPMs because no immediate indication of intent). Imagine native feed advertising volume with keyword-like CPMs.
Not that it's highly relevant, but I'm a developer of a popular reddit app for iOS.