For examples of why not, I recommend you read the rest of the discussion. If you have, I recommend you actually address those arguments so that we know why you think them inferior to your own reasoning.

Personally, I might find it somewhat annoying when someone doesn't format his or her text worth a damn, but that sort of inattentive, lazy disregard for readability is a pretty clear indicator of the sort of personality of the person behind the text. It provides an easy, first-glance means of deciding what nodes to skip over when I'm feeling impatient or otherwise unwilling to muddle through someone's half-baked natterings. Like well-formed code, well-formed text takes care. If someone can't be bothered to try, I'm not always going to have the motivation to bother trying to read what he or she has to say. I'm fine with that.

I don't want my own options for how I format my nodes to be limited by the willful ignorance of others. Let's not dumb down PerlMonks to cater to the least common denominator.

How's that for a reason not to have linebreak tags automatically added by the PM scripts? If that reason doesn't work for you, there are several more reasons in discussion above your comment.