You don't wanna be driving around without doors on. Most places require you to have a lip so your feet & anything on the floor of your vehicle doesn't roll out while driving, plus you would have to mount mirrors on your a-pillars. And those doors provide a lot of structural strength for the cab while you are driving.
I know on either this thread or one of the other numberous ones this has been brought up time and time again. Hit that search button on a couple sites and you'll find most of your answers. If I can find it for ya I'll post it here.