Blocking comments prevents harassment. I've seen people blocking harassers, the harassers would continue to harass them, and the harassed person would know they still comment (the interface shows "2 hidden comments"), and that anyone but them can see those comments.
Actually, just blocking them from being able to respond directly to comments between blocked users could be pretty useful.
If wanted the person could make a separate comment fighting perceived inaccuracies if so felt needed but would likely look petty in most cases and dissuade the practice.
I wonder what downsides could happen but still sounds pretty good if complicated an option.
Its utility isn't meant to make you appear as if you don't exist, but rather eliminate the ability of the blocked account to disturb your experience in any way.
If you block someone, but they can still leave a bunch of nasty comments on every one of your posts which are still visible to everyone else, then that goal hasn't really been achieved.