yeah, this makes sense, and of course it might make a difference, especially with a "good placement" in poor rock, etc.
My point, and I think we agree, is that if you place a piece and think, "this is marginal" (for whatever reason) and then you place a screamer on it, it's still possibly marginal, even if the screamer ends up helping. You wouldn't (or at least, I wouldn't) act as though it were suddenly a good piece -- in my mind, it would still be marginal and I'd want to get a good piece in when I could.