Ron,
I don't know what the thinking patterns were 100-200 or more yrs. ago but I offer for your consideration:
The structural ridge resolves the thrust issue. Completely. But that's only important if there IS a thrust issue. If your tie is strong and close enough to the tops of the posts, that will contain the thrust. The queen post-principal purlin arrangement often seen in the old barns also contains some of the thrust with the queen post tie. Of course, with a queen post system the lower tie would be large enough to support the added vertical load. So why/when do we need a structural ridge? If you desire usable (walkable) space on the top floor (above the highest tie) and you go with something like a 4-5' post extension (knee-wall) then you have some bending issues in the posts. This changes inversely relative to the pitch of the roof.
I kind of like the 1-2' knee-wall with a 12 pitch roof. Throw in a couple of collar ties for good measure. A common arrangement in New England salt box frames. Gives a decent amount of head room on the upper floor.
But back to old barns--I don't recall seeing too many that made use of the space above the wall plates (in terms of another floor) so the structural ridge wasn't necessary--the tie was close to the top of the posts and/or there were queen posts.
These are just my observations, not based on any research or historic expertise. Form follows function. Happy Holidays to All!!
