it can't hurt.
with that logic, we might as well do away with airport screening to. consider that you only need a handful of terrorists boarding planes, in relation to the total number of people who fly every day - so why bother screening at all. why bother checking passports then, or visas. the "odds" of finding someone is so low.
That's a silly analogy. Everyone who gets on an airplane goes through a screening process -- that's 100% screening. If you search 5000 mass transit riders a day, that's only .14% of all passengers, which means there is a 99.86% chance that a person will enter the system without screening.
"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.