Dang, Tom, you're almost as long winded as I can be, lol! :p
I've seen shackles break before, but not on the trail, true. They break here, but under much more severe service. I have seen shackles break and straps remain intact. I think seeing what happens in some of the lifting situations here has biased me somewhat -- but the situations are not analogous to recovery, you're quite right. That "jerk" encountered in recovery is often missing in lifting, for instance.
Some of the reasons for the breakage around the mill I've seen may have been the use of a compromised shackle that was not inspected, or has damage ignored. Since I generally don't do rigging here, I wouldn't know about any particular instance.
So, okay, we'll disagree! :)