Thanks for your reply, William.
If I'm tying silk-bodied soft hackles, I generally just use waxed Pearsall's Gossamer silk... mainly because it's faster, and not too much different from using tying thread, besides the diameter, of course. If I'm tying soft hackles with a body of something else - twisted pheasant tail, dubbing, flashabou, etc -- I use 8/0 tying thread, regularly waxed (I do the Davie McPhail thing, and keep a blob of tying wax on the inside of the first knuckle down on my index finger). The smaller diameter makes for smoother tie-in points, I have more color options, and it gives a lot more choice regarding the size of the head (I usually prefer a pretty small head, but smaller diameter gives one the choice to go larger if desired).
Having said that, I'm using the buttonhole silk for the abdomen on this fly (I switch to Pearsall's for the body) for a number of reasons: Leisenring called for it, at least in the written recipe, with the admonition that the unwaxed thread becomes somewhat translucent (so I don't want to use the wax on the abdomen). And because it's a chance to do something different, to learn other methods and ways of tying, maybe expand the repertoire a little. Were I to tie it for my own box, I might do stuff differently - I'd probably use a dubbing loop, for example -- I tie a lot of steelhead flies, so it's something I can do reasonably well and quickly. But for this swap I broke out a Clark's block (of your own manufacture, kindly gifted to me last year by Ray) because it was a chance to play with spun bodies.
I love tying -- but my boxes are filled with largely utilitarian flies, partially lots of them are tied the night before an outing ("Wait! The grey foxes are hatching already?! Ugh. All I have are these are the ones I tied last year, and they're terrible!"), partially because my mentor is a commercial tyer, and probably partially because of my own rather particular character. Tying in a swap like this is a chance to play, to explore, to expand, with time and thought and investigation. To engage other ways of tying. And I like the slow process of research, and trying a bunch of different options - It's relaxing, engaging, and fun.
I hope you don't mind the long answer to a non-question! But it kind of explains kind of the rationale for this thread. Make the process visible, and all that.
By the way, you've already cost me $25 this week, as I ordered the second edition of Hughes' book yesterday. Looks like you're going to cost me more, as I am intrigued by the 14/0 Griffiths thread!
And thanks to you, and to everyone, for the extremely useful feedback. Keep it coming!