I guess the point is; if you need to weld in floor pans and you want to do a schmick job you could get to areas a lot better with the body off the rails grinding back welds etc. Also those body rubbers do perish. You can also turn the body to clean up the underside without the need to lay on your back and have your eyes fill up with grit etc. If it's a great survivor car and doesn't need any of this then not much point separating I guess.