is that there is a limited but large case of problems where randomness shouldn't help, and it's disappointing that academics in that field don't know this, and that the terminology they use lends itself to this confusion.

Which field are we talking about? What people? The weighted majority algorithm (the topic of the post that started this all) is one of the cornerstones of statistical learning theory. I would guess that pretty much everyone who knows statistical learning theory well already knows that pure strategies are optimal given complete (probabilistic) knowledge of the environment.