My wife was on the source selection team...Fort Worth one week, Seattle the next.

The pilots called your bird, Monica:cool:

Simple answer, Chris. Margin of safety. Notwithstanding the fact that we do not yet have jet engines that are powerful enough to create more thrust than the weight of the plane itself, the fact of the matter is that anything that takes off and lands vertically does so solely on the downward thrust of it's engines (or rotors driven by the engines). If there were to be a problem with those engines or the control system needed to control the aircraft during vertical flight (and inevitably there will be at some time), the aircraft would be left with no options and would simply fall out of the sky and crash, taking along all of the 100 to 800 passengers to their doom.

What many layman don't realize is that the WING on any airplane is what supports the aircraft during flight, from liftoff until touchdown. Thrust produced by the engines simply gets the airplane from point A to point B. If any airplane loses all of its thrust, the airplane will simply need to be glided to a landing spot (a slight oversimplification, but basically that's all there is to it). The margin of safety is almost immeasurably higher, which is what public aviation is all about.