I think lots of people want X skills as high as possible. In this case I think they should conisder what is the level they would like the elastic effect complimenting skills at, setting a goal for those skills is important, then train those skills to that point and at last work on the desired skill.
you shouldn't forget that you train your skills even secondary, so it would be useful to stop earlier and make the rest of the training through secondaries.
PS: I believe with the train example, you would be faster driving the car late or you made a longer distance ;) But for training speed where you always train at 100% in the skill, and not driving with 50% it is equal.