Gotta skew the sizes some but for selling the average customer a mtb it works till you hit the tweenish area. Even in bike shops, 12, 16, 20, 24, and now even 26 inch bikes don't really come in frame sizes. Gotta hit that 27.5" before the frame discussion breaks out. Outside the basic kid's bike/mtb, the conversation gets fun. Even BMX you have micro, mini, jr which are all 20" but not the same 20" as expert, and pro and with none of them do you discuss wheel size or seat tube as the determiner for proper sizing, it all comes down to top tube length to really screw with sizing.
For real fun, my "tween" at age 11 had several 24" bikes, the mtb had a 507iso, the road bike had a 540iso (why raleigh used this I don't know), a 520iso track bike and a 547iso school commuter, yay for sizing.