My take on why you don't see too many folks using jib leech telltales is this: Sailmakers never put them on new sails, so no one knows any better. I never did and like you Quoddy, I've seen very few boats use them.
There is a ton of information and instruction on how to set up and use the jib luff telltales... for steering and trimming... but there is very little about using jib leech telltales.. how to read them and how to adjust, unless, of course you plow through the Arvel Gentry stuff, is pretty dense unless you're an engineer (sorry Rich). When you purchase a package of telltales... the instructions don't say anything about putting them on the leech.
I've had quite a few sailing classes and never heard the instructor give any emphasis to jib leech telltales...
That said... I'm going to put them on my headsails and see if it improves my trim technique.
I know that it will help me see airflow attachment but what is the correct action to take if they're not flowing properly.
I would certainly appreciate hearing how some of you advanced sailiors use their jib leech telltales.