Users should own their comments and should be able to delete them as they see fit. Sites like Reddit should not be used as a permanent or reliable source of information.
The problem is while the user is moving the comments are not, only deleted
Which will probably damage another user more than hurt reddit
Besides the user might delete things that would be interest read years later
Should be mentioned the rights are retained by the user. Reddit only gets license to reproduce that content and allow others to do the same. Although also being perpetual, irrevocable, it's similar to ownership cannot say they're the same thing.
I can totally respect someone trimming all their online footprint to avoid that.
If you want to retain your online anonymity, you have to be thoughtful about what you share online.
There's another kind of privacy where your cousin sees your reddit user id and looks up your comment history and finds out you are [_____insert secret here___].
Deletion of your history is a protection against the second kind.