In my personal experience the driving line gets most folks into a bad habit of where your eyes are looking. It prevents you from keeping your eyes up the track far enough ahead. Just my opinion though.
I've tried it with and without the driving line, and I'm faster with it turned on. This is partly because I don't have every track memorized and it's partly because I don't practice enough. From my point of view, here is what the driving line gives you:
1. Regardless of the speed of the car, it gives you a starting point for where to begin your braking during testing.
2. Like any other visual marker, it gives you a fixed visual cue that you can use for reference for setting your braking point.
3. The game automatically moves these markers as you go faster, slower or have tire wear. That's a huge advantage.
The negatives of using them are:
1. You have to force yourself to ignore the driving line itself as that is not always the fastest line.
2. You cannot see the driving line in tight traffic and will miss braking points sometimes.
3. It's really a crutch and hurts you when you race without it.
Workarounds for some of the negatives are:
1. Take a different line so you can see the driving line in traffic.
2. Double up on the visual cues. Use a marker on the side of the road in addition to the driving line in case you're in traffic. (I'm working on this one)
Here's how I use the driving line. I start off braking at the indicated braking point, especially when the tires are cold. This is 99% of the time a safe place to start, although every now and then the game gets it wrong by a few inches. But you can correct that. As the tires warm up, I begin to experiment with going deeper on each corner. I then compare the data in the data analyzer in practice mode looking at corner exit speeds. If I'm faster, I go deeper. If I'm slower, I back up the braking point a bit. Then when I have it all dialed in and I've tweaked each corner as much as possible, I just do laps trying to memorize the offsets from the driving line. This corner I might go indicated braking point plus a millisecond. This one I might remember as "deep". This one I might remember as "back it up a bit, be early". Together, the sum of these little cues becomes my lap plan which is what I was doing last night.
It is very challenging to use this technique when you're on someone's bumper. But honestly, that was just poor decision making by me last night. I was pushing way too hard the second half to catch Turbo and Phreak. I was over driving the car. And with one lap to go, I was betting the farm to be in the draft on the final straight. I got it wrong, cost me a run at the front. But still, lots of fun.
MX