"it can't hurt."
Yes it can. Wasting time and resources on pointless feel-good measures like this means that there are fewer resources available for more useful security measures.
like what? in this case, these are mostly beat cops doing subway checkpoint duty - they mostly write summonses of one kind or another most times. even if they aren't checking bags, having a presence there and simply looking for suspicious people is more beneficial then flagging someone for parking 10 minutes too long at a meter.