Well, I don't quite feel the urge to go looking for that thread.
The typical mouthwash contains a mix of irritants. It should be obvious that such a cocktail is likely not good for sensitive tissue. While I don't believe that commonly available commercial mouthwash presents more than a miniscule danger to people's health at large, there is not reason to unnecessarily expose oneself to even as much as the drying out effects of mouthwash that contains alcohol. Why use it if you can avoid it, especially if there are better options? Disinfecting your mouth is a bad idea to begin with, because you will always wipe out the beneficial bacteria, the first line of defense, with the bad. And guess which kind of bacteria will overwhelmingly win and reproduce in the end?
Triclosan and other antimicrobial substances: only a completely blissful ignoramus will put that pesticide willingly in his mouth (mouthwash/toothpaste) or anywhere else (deoderant).
Sodium laureth sulfate may be in mouthwash: it is an irritant to tissue
Alcohol: will over time dry out mucous membranes and may cause dry mouth syndrome. Dry mouth syndrome leads to rotten teeth in mouth-breathers! Dry mouth causes bad breath. Bad breath will lead to increased mouthwash consumption. That's great for business!
Artificial sweeteners like saccharin: no matter how little you end up swallowing, it may be of a concern.
Preservatives: most of them are questionable
Alternatives exist. Most people are just way too lazy to read the labels and to inform themselves. They prefer living in blissful ignorance.