I have no problem with the rule. Middlebrooks can't do anything there but he is still obstructing the basepath whether its his fault or not. You can't really blame the runner for being a little inside. On the replay you see him watch the ball and then turn around to run where he immediately trips over Middlebrooks before he completes his first step. It's not like he deliberately veered off the basepaths. The rule does have merit because you know that without it there would be some schenanigans on the basepaths. Crazy week for Boston sports fans though.