Three angles:

The technical purist: you’re modifying a URL in a way that, while in line with accepted custom, is technically incorrect. URLs should (the least effective type of should) basically be treated as opaque.

Social: it’s tracking stuff, sibling comment trees are good, I won’t reiterate.

Clutter: it’s getting in the way of the bit you should care about, and contributing to normal people no longer caring about URLs because they’re too hard, too complex.