If you have a post that warrants that response, odds are you were out of line in the first place. You shouldn't be judging generic pop artists when you stan for other generic pop artists. This a pop forum, it's never that serious
I would never say it but it's sometimes slightly funny depending on the situation. Most of the time I think it's dumb since whoever's in your sig/avi doesn't mean you're not "qualified" to comment on something.
It's valid when people are pretending to care about something they completely disregard or overlook as an important factor when it comes to their own favorite acts. It strikes me as disingenuous and yeah, I don't care to entertain the opinion of a person that is pulling things they aren't genuinely concerned with out of the air just to play contrarian or be obnoxious.
I don't use that saying but when I see others use it, it can be at times funny/relevant depending on what the situation is. It also depends on what's on the sig, whether it be artists, a statistic, or a quote.