I agree - it's not a perfect test, but it is easy to implement and costs nothing extra. I suppose it's more likely to catch those people who have been coerced into carrying something illicit rather than experienced smugglers who do it all the time.

It's not used to establish guilt, but to establish whether the person is worth further examination, so false positives (e.g. due to language difficulties) would just lead to either further interviewing or searching.

There's not really any good reason to not do it as the questions are not overly intrusive.