ducksterman,
I am not answering for Jack, but, a guide/fly shop owner friend of mine here used thread furled leaders all the time until I gave him a furled leader made from .005 diameter Coats & Clarke 100% Nylon Quilting Thread to try out. Now that is the only furled leader he uses. He claims it works just as well as the thread furled leader and he treats it with floatant just like the thread furled leader. He does not have to worry about mildew if he puts his gear away wet and he thinks it has more stretch than thread which saves his tippet on a hard strike. He also states that sometimes his thread furled leaders would get frayed on a snag and the nylon ones do not.
Now when it comes to why Jack does not use them, it may be because he has made 2 trips to Tennessee to fish with me and I may have converted him into a nymph/wet fly fisherman!
Warren
Fly fishing and fly tying are two things that I do, and when I am doing them, they are the only 2 things I think about. They clear my mind.