The primary reason to have horizontal dropouts is to discourage too much power output at startup. If set up correctly, with the proper quick release (preferably a blingy external cam version), the rear axle should slip slightly so that the rear wheel locks onto the chain stay and dumps the rider.
Modern dropouts, or modern thru-axle holes, emasculate the cycling experience by removing this simple reality check, and this is why such bikes are disqualified as vintage.