I love horizontal dropouts, to me, they are the epitome of design tweaks over the life of a longstanding product resulting in multiple benefits.
They aren't really horizontal, but the angle of them has evolved to be tangential to the rear brake pad mounting bolt. This minimizes as much as possible the amount of both angle and reach adjustment needed at the brake pads when moving the wheel through the range of the dropout.
The additional benefit of their long evolution is that the ride characteristics promoted by the position of the wheel within the dropout are complementary. If I have smaller, lighter tires on the bike and move the wheel to its most forward position, the wheelbase will be shorter, which generally will promote a quicker handling ride. Again, due to the angle of the dropout, this position also results in the rear end of the bike moving up several mm, both raising the bottom bracket and effectively steepening the head and seat tube angles - all characteristics that will make the bike respond faster to inputs.
Conversely, if I wanted to soften the ride, the first things to do would be to mount bigger tires on the bike and move the wheels back, which will increase the wheelbase, lower the rear of the bike (assuming same size tires front and back), slacken the head and seat angles, and provide for a bit more clearance around the rear brake and behind the seat tube. If keeping similar sized tires, this position also allows for more room for fenders for converting a bike to winter training. Again, changing all of these attributes simultaneously is complementary to the desired result.
