I know I haven't played HBD yet, but I have to say that anyone who wouldn't sit his switch-hitter against lefties in Mike's example is a fool.
If a guy has a .500 OPS vs. lefties through 100 games, it would stand to reason he would fare the same through the last 62. Ratings play a role, yes, but you all seem to be ignoring the fact that opposing pitchers and teams, ballparks, etc. all factor in. If a guy is hitting that low late in the season, I'd rather believe there is SOME reason for it and make a change, then stubbornly assume that his ratings dictate everything and his stats will change, and watch my team go down the crapper.
"He SHOULD, therefore he WILL" is some of the most flawed logic in existence, no matter how you justify it.