Why not? 'This' is just like 'me too' or '+1'. By itself, it's a waste of a comment, but since harlanlewis elaborated, the "This." was just a prologue to the rest of the comment.
If the number of votes for a comment were visible people probably wouldn't feel the need to annotate their comments with "this" or "++1". So blame the commenting economy of HN. Or just accept it.