I think the tree-force-model has come from the aircraft and wings designing department.
Since both drag and lift are equally subject to airspeed and angle of attack for a specific wing or entire aircraft, it does make sense to calculate with one resultant vector. Otherwise, you would have to calculate lift and drag seperately and the put them together again. For all the graphs it's much easier with one resulting force.
But in the end, both models still describe the same effects, don't they? (Yes, it does fly - and yes, you need some energy to keep it doing so)