Almost all Light Sport Aircraft, use mph. Experimental GA too. And, as you pointed out, some older aircraft. Together, these are a large percentage of the overall fleet.
I would suggest that iFly just use fractional knots internally. It doesn't have to display them or allow the user to enter them.
For those reading this who're wondering what Johnny is talking about: try editing an aircraft profile and putting in 80 mph or 88 mph as one of the speeds (such as Cruise Speed). It gets converted to 81 or 87, respectively. I pointed that out in the Beta Users forum as a "quirk". Brian replied that its because iFly keeps everything internally in knots, and asked the Beta Users whether they thought speeds should be enter'able in fractional numbers so iFly would not just store numbers as integers (and since it's storing them as knots, it's losing precision when converting from/to mph and therefore displaying the wrong numbers).
t's not a big deal. But it's weird to have the software not display what you entered. I don't think there needs to be the ability to enter a speed as 77.7 mph (or knots), versus 78. What POH lists speeds out to the decimal points? But that doesn't preclude the software from using floating point rather than integer for the precision. Either that or keep a flag to indicate whether a number is stored as knots or mph.