Irregardless, Awsure,
Jeremiah and BootCamp make salient points, I'm sure don't disagree. BootCamps analogy speaks to a point I was going to make, and that's not only does the roof add structural rigidity, but so does the windshield and backglass. A lot of people don't understand how much structural stiffening is provided by the glass in an automobile. This is like the PickUp truck with the tailgate down to improve gas economy (dis-proven but not totally done well by myth busters..I believe them...but think they could have done a better job), after a while you'll see the beds (or at least on much older trucks) get rickety, in this example the tailgate is the metaphor for the roof.
I'm babbling. And now I'm making redundant observations. And now I'm looping.....and now I press the OK to submit this post..... must stop.