I have been told to PLEASE report these - by police officers. They don't go get the guy necessarily, however if a cop pulls the person over at some point in the future these complaints do show up on the computer and the officer is less likely to be kind to the driver and give him a warning instead of giving him a ticket! If a driver amasses a couple of complaints, he may be sent a letter by some departments. Some police departments won't even take a report at all. Just depends on where you are.
If you CAN resport it, do. It may help statistics too...