Quoted from SkaterVet:
I'm curious....I've never given single vs double vs triple coin slots much thought. I've also never entered more than one coin at a time. Does anyone know why the majority are double? Is it for the aesthetic? Or, is it just in case one coin slot malfunctions? Some other reason?
Way back when it was because you could put nickels, dimes, and quarters in a game, hence the triple slot. Then, when it was just quarters, went to dual on some for fault tolerance. Then the dollar coins came out, so needed a slot for that.
Also, the coins doors can be used in many different countries that may have differing coins available, so maximum flexibility in getting people's spare change is a goal.
The aesthetics wouldn't matter if everyone had been used to the single style. Then multiplies would look weird.