> but clearly a common practice anyway

Well, people want software from somewhere. One of the points I see iterated from Arch users (including Arch developers) is that there's a very large array of software available—but if you exclude AUR, you're at ~15k packages, which is far less than e.g. Fedora or Ubuntu (both around 60–80k). So how would you install any given package that's outside this smallish set, if not from AUR? It's not a surprise that people do so.

AUR is in this very weird quantum mechanical state where proponents like to tout all of its advantages but are very quick to disavow it once anything bad comes up; I guess you can't have your cake and eat it too?