I am guessing that people do this mostly to make sure that they as the receiving team don't have to mess around with that ball on their side of the court or in their pocket... but I've never asked why they do it. They just do, and for me, it's a "when in Rome..." situation.
But as you say, people do it because other people do it, which means it can't be that important.
Also, I think people disregard it because it doesn't clearly impact play like so many other Code provisions do. Hooking, foot faulting, calling lets, giving benefit of the doubt . . . all of those affect who gets a point. This rule -- which does not even come with a remedy -- does not affect who wins the point. So folks kind of figure it doesn't matter.
Another reason why people ignore the rule is because it does feel good to get a mulligan, right? The serve is obviously out, so you get a free "practice return." Even if you don't rip it, you get to hit a ball that isn't in play. That actually doesn't happen often in a real match, right? I mean, can you imagine if the opponents sent the balls over to my side and I took a practice cut at it? So it's tempting to treat an obvious fault as a chance for a free swing.
Anyway, OnTheLine's experience has emboldened me. I'm going to start mentioning this to obvious offenders, be they opponents or partners.
I'll report back here, assuming I don't get strangled.