This is a topic covered all over the interweb. Basically, having photographed a bunch of accidents over the years, the only time doors seem to add anything is in a side impact. The latches for doors aren't meant to keep the unibody from folding like a taco. And if ANYONE thinks the window frame adds ANY structural rigidity, go out and see how easy it is bend that thing over... Does a door add a crumple zone, I've seen about 50/50 the door crumples with the front, or just flat breaks the latch and swings open.
Hey Scott, the door being tough to open when twisted, is only limiting your flex!!!!
OF course, this is only my experiences covering all manner of vehicle accidents. No, I don't have pics since there are legal issues. And, I wouldn't mind running my ZJ around some with the doors off...