since the main engines wouldn't have to carry around half the equation it'll mean much higher thrust to weight ratios
on the other side you need energy to wastefully slow down with respect to the aircraft frame (i.e. provide delta-v to) 80% of the outside air - nitrogen - you scooped. Engines without slow down of the air mass - scramjets - are different beasts and have their own issues).
Thus you have choice - carry with you and speed up 1 lb of O2 to the speed of 5 Mach or slow down from 5 Mach (slow down is the same energy consuming action as speed up) 4 lb of nitrogen and 1 lb oxygen of the outside air.
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on the other side you need energy to wastefully slow down with respect to the aircraft frame (i.e. provide delta-v to) 80% of the outside air - nitrogen - you scooped. Engines without slow down of the air mass - scramjets - are different beasts and have their own issues).
Thus you have choice - carry with you and speed up 1 lb of O2 to the speed of 5 Mach or slow down from 5 Mach (slow down is the same energy consuming action as speed up) 4 lb of nitrogen and 1 lb oxygen of the outside air.