I'll offer a tidbit that will get me in trouble, perhaps.
Something I tell the students that come to our workshops is that I've seen a good number of old barns that have sloppy joinery. These buildings have been standing for a long time through big wind and who knows what else. They stand because the roof and foundation were kept up, and plenty of well cut frames have gone down the tubes because of poor foundation or no roof upkeep. I encourage the folks who take our courses to work carefully, but I beleive timberframes can be stong without high levels of precision.
It could be that after a certain level of tolerance, it's all for looks.....
