single engine isn't a benefit for this design either
and yes, the Osprey can run on a single engine. part of the complexity is the wacky transmission linkages required to transfer power in the event of a flame out.
No one's going to build a man-rated single engine military helicopter when a dual engine one gives them redundancy against battle damage. Plus, aren't all aircraft that are expected to operate over oceans (Navy, etc) *required* to be dual engine? Notice the Apache is still dual engined, So is the Navy variant of the AH-1. Neither is your upcoming Tiger ground support version, or the Russian Hind series.
The devil is in the details, and this is no Cheyenne. The ability to modify the rotor speed of two coaxial rotors, with serious rotating gyroscopic mass each, is a big deal. To some engineering ignorant hack (the type who thing "black boxes" are made of some super material to survive crashes) then it's no big deal.
But to us aviation nuts, and RC model (err...drone? RPV? UAV?) pilots, who've dug into this sort of thing since we cut our teeth on old modelling magazines, this prototype, and the CarterCopter are very exciting indeed.