I'd suggest at most depth 1 to anyone. 2 is way too broad I think but I agree that many are using it just because it's the default.
Using Trust depth 1 would effectively remove DT2. I don't think that's desirable.
I mean custom trust lists. For users who rely on DefaultTrust, depth 2 makes sense but only because DefaultTrust inserts itself at depth 0... which is clunky in its own right (makes combining DefaultTrust with custom inclusions/exclusions too complicated) but that's another story.