Originally Posted by mstateglfr

Horizontal dropouts- in addition to the already mentioned ability to run single speed without tensioner and your stated theory of the ability to effectively adjust chainstay length which gives you different handling, a very real benefit to horizontal dropouts was to account for builder error. It wasnt some epidemic or anything, but horizontal dropouts gave the builder a wider tolerance with regards to how aligned the frame was. If the rear triangle was slightly off in one direction or the other, that could be accounted for when setting the tire in the dropouts.

Tires- Continental GP tires arent 330tpi. They are 3 layers of 110tpi. Conti chooses to advertise that as 330tpi to make it seem like a higher number(and it works, clearly).

Wheel slipping in dropout- this doesnt happen with an internal cam QR skewer, which is the only style of QR that should be used with horizontal dropouts. The internal cam QR has a significantly stronger grip on dropouts than an external cam, so it doesnt let the wheel move at all. You can smash cranks creating absurd amounts of torque and not make the wheel shift.
The only benefit of external cam QR skewers is weight. And using the stock external QR skewers on most bikes will save you maybe 40g compared to Shimano Ultegra internal cam QR skewers. There just isnt a realized benefit for external cam.