Another shibboleth that distorts baseball statistics says that when one runner is retired on a batted ball and a second runner is safe only because a throw is off-target or is dropped, no error will be charged. This affects, and distorts, three different statistics. First, the fielder who makes the bad throw or drops the ball does not get a blemish on his fielding percentage. This is least important, because who really cares about fielding percentage unless there is something like a 150-game errorless streak at stake. Second, if another runner scores from third on the play, the batter will get credit for an rbi, which he wouldn’t if the double-play were completed. Third, and this is the big hit, the pitcher’s ERA will be responsible not only for the runner who is safe because of the errant play, but for all the runs that wouldn’t have scored had the second out been recorded.
Are these results just? No. If a first baseman drops a throw, it shouldn’t matter that an out has already been recorded. He is just as responsible for catching the ball, and if he fails, it should be his error. But maybe the second baseman who has to pivot in the face of an onrushing runner and relay to first shouldn’t be responsible if his resulting throw is off target. But hey, he is held responsible if his bad throw allows the batter to go to second. And some pivots are harder than others. The official scorer can surely identify when a bad throw is caused by the difficulty of the play, and when it is simply the fielder’s fault. This is the same judgment he makes on any hard shot or any slow roller that the fielder flubs.
From the batter’s point of view, if he is up with the bases loaded and hits a routine double-play grounder to short, whether he gets an rbi should not depend on whether the first baseman drops the relay throw. He didn’t do his job and he shouldn’t be rewarded. Conversely, the pitcher who does his job by inducing a double-play grounder that should end the inning shouldn’t be stuck with a lot of earned runs just because the first baseman drops a throw.
In short, to say “you can’t assume a double play” has no basis in logic or practice. Any error assumes that an out would have been made if the play was properly executed. Given the right circumstances – if the shortstop or second baseman has the time and space to get off a decent and timely throw to first – there is no reason not to assume the second out, as well.