Mainsail Telltales

At, the author Shevy Gunter of "A Truer Tale of Telltales" strongly recommneds using telltales along the luff of the main like we do the jib.

Has anyone used such telltales and how have they been useful or not?

The reason as I understand it from my reading is that the luff telltales, positioned about one foot back from where the leeech of the jib falls on the mainsail, is that it tells about the air flow over the luff of the main, which in turns tells us about the position of the jib since the jib alters the path of the air for the main giving it more of a header. Maybe I have misunderstood, but that's what I think I've read!