I like vertical dropouts. The principal advantages to me as a rider are somewhat easier wheel changes, and not needing to tighten the quick release significantly, since forward pull by the chain is resisted by the dropout.
So, why might they have taken so long to catch on? First, they require that the frame-builder work very accurately in both dimensions (exactly equal chain stay lengths; exactly equal seat stay lengths). For those who aren't jigged to do it precisely, there may be more ways to get embarrassed. Second, particularly the British were coming from a hub-gear and fixed gear tradition, which required the horizontals to compensate for chain wear and different cog combinations. Heck, I remember early 60s Raleigh SuperCourses, native 10-speed, with the narrow axle slots characteristic of hub gear bikes with flatted axles (One buddy made high-strength chrome molly replacement axles in the student shop, since the flatted axle was such a dumb idea and bent so often). So, the biggest "con" for verticals remains the difficulty of using with anything except derailleurs.
How much do I like Verticals? Enough that I made, and later had others make, custom fittings with gear hanger, to convert curved-seat-tube Schwinn Town & Country tandems to verticals. This allowed running 700c rear wheels instead of 26". Did between 6 and 10 of these conversions over time. harvey sachs mcLean va
<< I recently acquired a 1973 Jack Taylor Tour of Britain (#6559) that has Campy vertical rear dropouts. They are clearly original. I've learned from the archives that campy introduced them some time in the 60s. Also that Raleigh used them on the R.R.A. in 1973 (same year as my JT). So, questions: Why vertical rear dropouts in 1973? Did they have some special advantage that made them better for a specific discipline (i.e. TT)? Why did they take so long to catch on?>>
<snip> Peter Weigle ...informed me that they were the preferred dropouts for touring frames as they were stronger than horizontals and made wheel changes, and therefore tire repairs, easier.