Sure, much of the time removed comments are poor quality.
Sometimes though, they are comments that ought to have been the top comment.
Anything that gets flagged will be seen by far, far fewer people. Most people here don't have shadowdead on. Most don't even know that it exists.
And that's just comments. More important are the stories that get wiped. Stories that are flagged by motivated minorities at lightning speed, unseen unless you're obsessively browsing new. Even if you do find one, you can't comment on it. One vouch isn't enough to bring it back, even if you see it shortly after it's posted. Moderators can cite "the will of the community" and there's nothing you can do about it.
Stories don't even have to be deleted. A post that gets flagged for just a short time can drop impressions by 90+% and never get back on the front page. This happens a lot with certain topics.
So, you're right in what you're saying - but that's far from the full story.
It would actually be interesting to see the numbers: how many registered HN users have showdead=yes?
FWIW I not only browse with that on, but also vouch for comments that I feel were killed unfairly. I've seen dead comments revived (and then upvoted) more than once.
