Does "it doesn't tell you anything about the car's location" refer to lane position??? In that case, I can see the point if you're on a multi-lane roads ... I do find it challenging when taking the right lane of a 4-lane road (without shoulder), and not know if that car is in my lane or the left lane (I don't ride with a mirror). OTOH, I know how close the overtaking vehicle is, no matter which lane, and that is helpful!
Lastly, I will echo (repeat?) the point that riding a frontage road next to a highway is frustrating. But I've enjoyed a laugh a couple times riding an empty trail and getting a Varia alert .... look back to see there's a train on the tracks parallel to the trail (N Cedar Trail for the MPLS folks).