[varenc]

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.

[wat10000]

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.