A quadcopter cannot maintain flight on three motors. You loose one for any reason during flight, and it tumbles to the ground. It has the glide ratio of a rock. That's where the real benefit of a hex or octo come in. From what I understand, once you have six motors, you have enough redundency to maintain flight to land if one is lost. I believe you have to deal with yaw issues becuse the torque is out of balance to the point that it can't be entirely compensated for, but flight can continue, and it doesn't necessarily fall out of the sky.
To me, this is a MAJOR flight safety factor. When it comes to quads, no matter how much regulation is applied in the name of safety, no matter how thorough a pre-flight you perform, no matter how safety conscious a pilot is, and no matter how perfectly a pilot controls his aircraft, there are going to be mid-air failures, and quads will occassionally fall from the sky onto whatever is below them. And the pilot may have done absolutely nothing wrong. Electronics can simply die. ESCs are electronic. Props are stressed and fatigued by flight forces, and can/do break now and then, having shown no signs of a problem during pre-flight. Bolts looses from vibration, and structures separate. It will happen, eventually. This is why is is so important that quad pilots in particular never fly in a state where the quad can't fall at any given time. I don't know about you guys, but I've met Murphy... and he can be one mean dude.
