Is it worth learning to drive with driving line off ?
It depends on what your goals are. If you're simply interested in racing with total disregard to your own skill, use the line if you like. If you want to become a better driver, you need to take off the training wheels and run the car on your own without the game telling you how (turn the line off). Real-world drivers don't enjoy a magic, glowing line drawn on the track by God, but rather they have to use their own judgment to figure out how to best get their own car around as effectively as possible. If you want to really be a good racer, you need to go with the line off and accomplish everything on your own.
Also, the driving line isn't perfect. It's sort of a generalization. Different cars perform differently, situations vary, and I think the line tends to play it pretty safe. I sometimes find myself taking a different route with a slower car than I would in a faster one, or taking a different line with a more agile car than I would with a less agile one. The line might help you get around alright, but probably not in the most efficient way possible for your specific car and setup.
I don't see a problem with leaving the line on if you want...
The line's fine if a player needs that help, but if they want to stand on their own without the game holding them up then they need to take off the water wings.
If you have problems remembering the brakepoints, just remember that the gear indicaotor will start flashing.
I don't make much distinction between the flashing gear indicator and the magic race line. I'm sure there's race cars that have similar features built into them, but I would think that such a crutch isn't so common in motorsports and again if a player really wants to improve their skill they need to stop relying on the game helping them out all the time. As before, if they need the help, want the help, and are happy with training wheels, more power to them, but I also like to encourage players to eventually take off the training wheels at some point.