In most states, you can be arrested for DUI if you are in the vehicles driver seat with the key in the ignition. No need to wait for them to swerve at a few targets.
I would be more in favor of a proactive approach like if the undercover guy sees someone that is obviously drunk head for the door, stop them. Sitting in wait smacks of entrapment.
That is how I reason it because it is a public safety issue. If the police knew someone was going to abduct a child, commit a rape, rob a store, or commit a murder they would stop him. It's public safety. I think that if someone is obviously impaired the police have an obligation not to let him get behind the wheel and drive.