I am disappointed with some of the comments in these posts. It is fine to disagree and state your point, but most of the arguments above ignore simple facts and in some cases, are downright rude.
It is not a bad idea. It is an idea that has merrits and detractors. Just stating it is a bad idea only proves you have not thought about it.
If you think it would take too much time, or would be distracting... then don't use the feature. It would not be required. Plus, it could be limited to offline racing only.
The "it is not in a real car" argument cannot be rebutted directly because it is a narrow view that ignores all the rest of the things we have that are not in a real car. It does not address the particular issue at hand (see below). However, indirectly, you can look at it as just the same as having a driving instructor with you in the car as you learn the track... something that is extremely common in a real car on a real track.
The "it is not in a real car" argument is valid if backed up with a valid argument. It is not a point unto itself. I think this argument could be applied to the racing line suggestion above. However, this is still an area of opinion that has varies from person to person. I would see the racing line graphic as being too much like a game, and not enough like a simulator. Plus, there would be other, more realistic ways of accomplishing the same thing.
There are so many things in LFS now that you don't have in a real car, it is hard to justify any argument based on that assumption alone. For example, do you have text pop up on your windscreen that says "Sorry, my fault" when someone bumps you on a real race track? In a real car, you can't communicate with other drivers at all until after the race. (Other than throwing them the finger).