If you apply the same weight consideration of tent vs. tarp (choosing the twice-as-heavy option) to the rest of the stuff in your BOB, the BOB will weigh a whole lot and will be an impediment to your bugging out, much like a boat anchor.

If weight is so slight a concern, then it is not really a man-portable BOB, it is a car camping rig.

People who don't believe that "ounces become pounds, and pounds become pain" have not put in the time under a heavy ruck.