From my point of view, the idea is not to catch people; it's to prevent the behaviour.
By raising the probability, or at least the perception among drivers that they are likely to be stopped, and if they have been drinking excessively, they will be arrested and convicted, is extremely important. That perception is something that has been served well by public information campaigns, even alerting people that there will be a barrage here and there.
It's not to catch people; it's to prevent the behaviour from occurring. As you mentioned, one way is to increase the sense that the probability of being arrested under the effect of alcohol is high.