I'm going to go out an say that horizontal dropouts are pretty useless at this point. If they aren't used for keeping tension (as in a derailer geared bike) vertical dropouts make wheel changes easier, don't allow for misalignment of the wheel, have no danger of slipping, and make fenders a lot easier.

If you want to run SS, fixed, or an IGH, then sliding dropouts are a much better alternative. They have all the advantages of horizontals (by which I mean the one advantage, they take up chain slack) but they can also be used with disk brakes, they don't require re tensioning after every wheel removal, and they just look much cooler.

I suppose I should confess, all of my current bikes have horizontal dropouts, and I have no real problem with them, but I can see how they are not ideal for all circumstances.