Not to say never, but I have never seen the case where having the axle at the back of the dropout makes for better shifting. It has always been the case shifting is better in the vertical dropout position.
I imagine the horizontal dropout is a legacy to when derailleurs were uncommon. There was no derailleur to pick up chain slack and the horizontal dropout allowed to pick up slack less than a half of a chain link. Try making a fixed gear bike with vertical dropouts. It is a dance between the chain, front ring and rear cog that usually doesn't work out well.
Agree with both points....and also referring to 65'rs comment, these things were in use for years, and it seems to me if a frame builder wanted to keep the axle in the optimal position, they'd have been spec'ing vertical dropouts a lot sooner. To me looks like a chintzy engineering bodge, and also speaks to the thought that the average owner has no ability to adjust things correctly.
Anyway, I suspect the consensus is correct in that the horizontal dropout was a holdover from the fixed/coaster/IGH days where some adjustability was needed in the drivetrain, and fashion (or fear of changing fashion) kept builders using them forever; and for those frames that didn't spec the high end adjustable dropouts, sticking an ugly spacer in was a suitable course of action (/Flame suit on)