I don't disagree a bit with what you've said.
On the other hand, I think a developer shouldn't have to be told more than twice that:
can eat up a couple hundred gig of spool without blinking, but that
GROUP BY item
goes by in the blink of an eye. You'd think that wouldn't be a major paradigm shift for them--but it is (okay, in one case I've been dwelling on lately). And it'd be even nicer to be heard the first time.
(This is an oversimplified example--the point being DISTINCT versus GROUP BY.)
They laughed at Joan of Arc, but she went right ahead and built it. --Gracie Allen