Part of the idea of all one size frame is interchangeability. The Flow frames are not going to be interchangeable anyway as they will not work for brood and I don’t want the bees overwintering on them because the queen will have no where to lay when late winter comes around. (too deep, wrong diameter cell).
When Langstroth invented his hive he was trying to make boxes out of standard size material and because anything deeper than 9 5/8" would have weighed more than 90 pounds full of honey, he did not go deeper. Since not everyone could lift that (even back in the day of 100 pound feed sacks) he also made shallows for supers. Dadant decided to make them deeper (11 5/8") and Dadant also made his supers deeper (mediums aka 6 5/8").
I’m not saying he is right or wrong but Dadant was the one who came up with the mediums. Here is what C.P. Dadant said: “The brood combs were altogether too large to be used in a super. There was too much danger of heavy combs breaking down, when full of honey, too much trouble in extracting. Besides a deep super seemed to attract the queen, when a shallow super did not. With the Langstroth brood chambers, the objections to a deep upper story were not so flagrant; yet they appeared to us quite sufficient to condemn it. There was often too much room at the time, so much so that a number of people who double the size of their brood-chamber, previous to the honey crop, often think it necessary to add the second story at the bottom, instead of the top. Then these combs were not so handy as those we adopted at that time, for extracting combs, with the advice of Mr. Langstroth. Shallow frames, such as have been offered by dealers, are too shallow. Those that we use are of the right size to be uncapped at one stroke of the knife and yet they contain nearly 100 square inches of comb, or over two thirds of the capacity of a standard Langstroth brood-frame. Although we have often heard beekeepers say that they could not tolerate two different sizes of frames in the apiary, we find less objection to these extracting combs than any one can find to the use of sections for comb honey.”–Dadant System of Beekeeping