If we used the actual units on gauges in the actual aircraft, we'd have to contend with kph and meters for some aircraft (German, IJAAF, Russian, Italian), mph and feet in others (USAAF, RAF, unless RAF was knots), and knots and feet in others (USN, and maybe RN?). Also, I'm not sure if IJN used kph or knots in the aircraft.
I've flown in games that had that, and it was really hard to switch around among aircraft -- at least for me it was hard to get used to. You can always do seat-of-the-pants flying, but if you want to check things like your speed, altitude, and rate of climb quickly in combat situations, you need to have the feel of all the different measurement systems then.
Also, if you know stats on aircraft and then want to know, as you are being chased by a Spit 16, say, "Is a Spit 16 faster or slower than the plane I'm in?", multiple systems make it more difficult to know.