Oh, absolutely; the proper (omnipotent) evaluation of any position should just be win, draw or loss. But as fallible humans (and computers!) we use terms like your "down half a pawn or so" to denote situations like "White can draw but he's going to have to play pretty well to do it." My point was that the "drawing margin" in chess (how bad a position a player can find himself in and still be able to draw with best play) is surprisingly large, even for grandmasters. Here's a striking example from last year, involving two of the top five players in the world: http://www.chessgames.com/perl/chessgame?gid=1602565