I worked for Bell Helicopter for 20+ years, mostly on the tiltrotor programs, starting before they won the XV-15 contract and leaving after the V-22 program was well underway. At the risk of oversimplifying, but in response to questions and issues raised elsewhere in this forum:
1. The transmissions that drive the rotors are interconnected by a shafts like a tailrotor drive shaft, so if one engine fails the other engine drives both rotors. However, in the Quantico crash, the flash fire that took out one engine "melted" part of the shaft system, causing a loss of control. That failure mode, and the one that caused the flash fire, has been designed out.
2. The conversion system that tilts the nacelles is redundent and designed and maintained like a flight control system. In the unlikely event it fails in airplane mode, the V-22 can be landed like an airplane. The blades will be destroyed but the landing should be survivable and the aircraft not heavily damaged, not unlike a wheels-up landing in a low wing propellor driven airplane.
3. The XV-15 was transitioned power off from airplane mode to autorotation and a 60 knot landing could be accomplished from an autorotation; it was successfully demonstrated on the simulator from airplane mode at 250 knots at 100 feet AGL. The V-22 either can't transition or it's too hard for the average pilot, but it can still be successfully crashlanded power off in airplane mode. If you're in helicopter mode and lose both engines, it'll probably be a survivable crash, depending on what you hit.
4. The Arizona crash was pilot error, pure and simple, with the pilot trying to correct at night on night vision goggles for being too high and/or fast on approach and getting outside the approved flight envelope. You can argue whether or not the V-22 descent capability is adequate for steep approaches to a landing zone, but a warning has been developed for the impending loss of roll control and recovery with nacelle tilt is almost instantaneous.
5. Strictly speaking, the North Carolina crash was pilot error in continuing to try a control system reset following a hydraulic system failure, exacerbated by a flight control system response that wasn't well understood. The V-22 has three hydraulic systems, any one of which can operate the flight controls. The aircraft was flyable if he hadn't kept hitting the reset, which he wasn't supposed to do. However, it was an understandable reaction to the situation and the flight control system shouldn't have been so unforgiving.
6. Marines wanted to replace the CH-46 with a new Boeing all-composite fuselage tandem rotor design until they flew the XV-15. Then they were insistent on a tiltrotor rather than a helicopter. After Cheney tried to cancel the program, Marines kept it alive through their excellent relationship with Congress and a lot of hard work by Bell and Boeing because they think its capability will let them do the missions they get called on to do better than any other aircraft type.
7. Why tiltrotor? Say you have friends being held hostage in the jail in downtown Cheyenne, Wyoming and you are persona non grata in the United States. The V-22, as specified, could make a rolling takeoff from a helicopter carrier in the Gulf of Mexico at dusk with 12 well-armed and capable friends and extra fuel tanks in the cabin; land outside Cheyenne, Wyoming at 5000 feet above sea level before midnight to let off the rescue force and dump the cabin tanks; rendevous with the rescue force downtown at one in the morning in the parking lot by the local jail; make a vertical takeoff with 24 people in the back; and be back aboard the ship before dawn, unrefueled. That's the Teheran hostage raid and no helicopter can do it in one night, even with inflight refueling.
8. Is the tiltrotor in general or the V-22 specifically fundamentally flawed? In my opinion, no more than the early jet fighter, with engine TBOs in minutes, low rates of climb, and big turning radii; the jet airliner, with the Comet catastrophic failures due to cabin structure failures; the supersonic jet fighter, with strange handling qualities and high accident rates; the Boeing 727, with crashes due to high sink rate approaches at low power settings; or a highly automated flight control/management system, with the next to last words on the voice recorder being "Why's it doing that?". The operational capability of the tiltrotor relative to pure helicopters or airplanes is real and of significant benefit.