I am starting to think it is time to change the name to singles just so the "it ain't a Thumper types" will give it a rest. With my friends there are two strokes mixed in with Thumpers on about every ride. So this is where two strokes fit whether or not it offends the name sensitive.
Weight doesn't make much sense to me because Japanese 250's weigh more than big bore KTM dual sports but are an entirely different type of motorcycle. If you set weight high enough to clear Japanese 250's you capture all of the EXC's along with LC4E's and probably the 690 Enduro.
But, every time I ride in a group displacement is how things naturally get sorted out. Little bikes can't hang with big bikes in the open and big bikes can't hang with little bikes in the snot. When we hit the road for mountain paved twisties the little guys tend to stick with the offroad program. When the little guys hit the AA loop the big boys either cringe and suck up the hell or take the bailout loop.
If we have to do it, displacement seems like the way to go. But, to me, the split needs to be 600+ or you end up capturing big bore EXC's and TE's with Japanese and Euro long range dual sports.
640E, MXC200, XT200