Door locks are a deterrent that increase the difficulty and cost of a crime. If your neighbor's house is locked and yours isn't, then you're going to be more of a target. In that sense they do provide security, but of course any lock might still be defeated.

Similarly, I can see how structured psychological interrogation, assisted by a polygraph, is a useful deterrent. The presence of moles doesn't negate all of its value. Just like having your house broken into once doesn't mean you'll stop using door locks.

The difference is that locks actually do work. They’re not merely a psychological trick, they actually do provide a barrier, even if one that might not be hard to defeat.

Considering polygraphs don’t work at all, I have to imagine that an equally effective interrogation could be constructed without them.