I get the theory, but assume more thrust per engine is required to get back to the original two engine performance? Seems a waste of energy/fuel for 95% of a flight.
I guess it is due to properties of sine and cosine - you'll get a significant yaw/vmc reduction for a minimal loss of thrust.
IIRC Blackhawk helicopter has a similar arrangement with the tail rotor - it is angled by 6 degrees (or thereabouts), yielding a significant lift contribution for minimal loss of side force/tail rotor torque