I personally find horizontal dropouts give me the ability to ride different tire sizes and with varying chainstay lengths. Useful on a couple on a couple of '80s bikes that will take 33mm cyclocross tires (aft-most position), or 25mm road tires with a tighter rear triangle. Believe me = I can tell the handling differences between knobby 33s and supple 25mm tubulars inched toward the seattube. For those who can't appreciate the versatility or tell the difference between a fat or skinny tire - well, ... sympathy for you all. Vertical dropouts = best reason for N+1+1. Enjoy.