This is a very interesting conversation and, well I can't help but come and add my little grain of salt...
I must admit that when I started to get more interested in calligraphy, I was not very attracted by the very thin hairlines of the "golden age OP masters". I thought - and still think - that they hinder legibility, especially for my european eyes. And I'm convinced that legibility should always be a scribe's first priority. I love reading too much to accept that I can't read the words written on modern artistic pieces of art, for example... I find that extremely frustrating ! I learned paleography (on dutch documents....) and that probably makes me more capable to decipher handwritings than the average person is, but I'm frustrated when I have to work hard when it comes to reading
.
That being said, I really began to get mesmerized by the light strokes of masters when I started learning advanced spencerian script and OP capitals with Harvest Crittenden.
Some things require great skills, and perfectly mastered hairline strokes are undoubtedly the result of great skill.
I can do light hairlines, even barely visible hairlines..., but the OP masters' hairlines have something that makes them so much more graceful and perfect. Just like Andy said : it's the combination of subtle shades and hairy lines, combined with amazingly consistent oval forms that makes those hairlines so perfect. The lightness of the stroke in itself is not that impressive... But the consitency of the "secondary" shades expertly placed on perfect spencerian ovals dividing each other in perfect places... That's just brilliant !
Also, I need to point out that OP actually requires very very thin hairlines in some cases : when the hairline has to cross the words or the capital shape in itself, legibility will not be as good if the hairline is too visible. There are examples of signatures in Michael Sull's volume 2 where you can see a hairline crossing the words, forming an S shape (horizontal) around the design. It looks beautiful in the book, but when I see other people trying the same kind of design, it's just not working out (most of the time). The hairline has to be very very light and perfectly shaped in order to be only seen as a decorative element. The level of skill needed is outstanding, and it obviously takes time to get there !
I don't mean to be rude, and I sincerely hope I'm not, but when I see Ken's spencerian, I can see that he does not like that script as much as he likes Copperplate. It looks great in its form, but it lacks the grace and, well lightness and inspiration, of the master's specimens... I think this might be about control : copperplate is done slowly and requires a great amount of control. I think drills are not very useful to be able to make the letterforms, it's all about repeating the same movements consistently (flourishing is something else).
For Ornamental Penmanship, drills are needed to get consistency and confidence. The most important drill is repeating the oval shape, in all sizes and directions, so that you don't need to think about the shape you're doing when you're writing, you can think about connecting the words, about spacing each stroke perfectly, and you can quickly come up with seemingly complicated designs without even worrying about the end result... Control is not as important as in copperplate, but practice, study and inspiration will make all the difference.
I could compare this to litterature : it would be the difference between a good book written with correct grammar and some inspired paragraphs and a book where words just seem to flow effortlessly, poetically, and perfectly...
This is what makes OP so fascinating to me, not just the light hairlines...