Not pointing fingers at anyone in particular . . .
Sure you are, but whatever.
As was noted - there are different objectives to working a horse on a line. While reading a book or watching a DVD is good to gain knowledge, working with someone who can actually complete the task at hand is a more direct way to accomplish the goal.
Agreed, EXCEPT...what Kevin said. The person you're learning from actually needs to know what they are doing as well. Someone trying to learn has very little way of knowing if the person teaching them is teaching them 'right'.
That's why a 'combination' of resources is best. Again, this is where a book/video from a riding master; DeKunffy, Podhajsky, Klimke, Kottas, Hempfling, Dorrance, Whitney, et al... can guide someone onto the correct path and thus help them identify a skilled mentor.
Then the skilled mentor can provide the person learning with the final key by observing, commenting and guiding the relationship between student and horse.
It is the fact that so many, who have instructors/trainers, are coming to these boards asking the most basic questions, that is most disturbing.
Where has all the horsemanship gone?