The current function assumes that there are no blank lines between replies. But that is not a standard. In most discussions where posts are extended to more than some phrases, users do put blank lines between replies, to make them distinguish both in read and in source editing. Let's assume that someone is not using the tool and tries to edit the whole section and imagine that those replies are more than a couple of phrases and contain markup such as links. Impossible to read and reply.
But it is not more than usability and accessibility thing, it is also technical:
If there is a blank line before a reply (R1) to a comment (A) when another user presses "reply" after A, their own reply (R2) is put before R1.
Also, blank lines are put between and after replies to prevent edit conflicts. They rarely happen if the posts are separated with blank lines (different blocks - merged by mediawiki itself) and if they happen they are easily resolved with the edit conflict view (and even more with Two Column Edit Conflict View).