Re: And now for something completely different!
Late response, but anyway...
The purpose of the rule is to prevent teams from creating "artificial" double plays. Say there's a runner on first, and the batter hits an infield pop up, and the defense lets it drop. Without the rule, the runner on first must advance to second, and potentially the defense can get both the runner (force at second) and the hitter (out at first) out by letting the ball drop in play on purpose and then fielding it. The infield fly rule prevents these types of shenanigans.