I think commenting on TL;DR will lead to poorer discussions because commenters are relying on the TL;DR being an accurate summary. It could well be that the TL;DR has an editorial slant from the person who makes the summary.
And often the point of stories on HN is not some tiny summary or moral, but a long article with many different points and data that are hard to summarize.
Also, I'd argue that reading a long piece (especially if it contains an illustrative story) is much more memorable than a summary like this.