Not forgetting that OEI really does mean one engine inoperative, i.e. completely shut down, rather than one just throttled back, which is a waste of fuel.
This may have other implications (e.g. only one electrical generator working so no IFR), or where where the aircraft transmission is severely torque limited when OEI, resulting in a badly reduced cruise airspeed. A strong headwind en route can then completely mess up the plan.