Is the line useful?
In my honest opinion, after you get to know a track, it shouldn't be. In fact, I believe that it'll harm your laptimes more than most people think.
That line is basically a rought guide to stroll your away around the track, if you're hotlapping using the line, you're not getting far.
Not to mention that it hurts my eyes, looks unrealistic, distracting, etc. It's by no means a question of coolness, same as the aids.
As someone raised the issue, ASM and TCS off, well driven car, will always be faster than with these off.
ASM slows you down on corners just because it thinks a bit of slide is bad. Not necessarily that way.
TCS slows you down, as it refrains your car to put the power down. With the right modulation of the throttle, it's possible to be faster with it off in 95% of in-race situations. Plus, having it off means you can adjust turn lines merely my making proper use of the throttle.
Back to the line, I almost never felt the need for it (only a few times on previous GT games, to know the track faster), because I find references on the track that are much more accurate to me than that fluorescent blue and red line hovering on the tarmac.
It's more realistic visually, it's more realistic in terms of driving, because you HAVE to know the track, and it's a lot better in a long term.
I compare it to driving school, and the difference between driving with the instructor and driving by yourself; sure, it feels safer, but you can never do as good as you'd do by yourself. 👍
The visual part is very important for me, I don't want anything that blows the realism of the game, so that's why I pointed that as a minus for having the line displayed.