Let's get the illusion that a rollover is not happening in a DE out of the way. Here's a low speed rollover at our back yard, Limerock. Notice what happens to the roof.
Now, if it were me, I would ideally like my roof to stay in place via a cage. If there's no cage, as is the case in the vast majority of DE cars, I would like something to give way so my head doesn't get crashed. I don't know about you but my helmet sits about 2 fingers away from the roof. Car seats are designed to fold back under pressure. If a bar is there, you just lost that advantage. That's why half-solutions are sometimes worse than the alternative.
The 3-point belt system is designed to work with OEM (folding) seats and airbags. Once that system is altered, either via a fixed back seat, a harness bar or a 4/5/6pt harness you've lost some of the advantages of that design.
I'm not saying don't it, I'm saying that one needs to understand what disadvantages are introduced by implementing anything less than a full-cage/fixed seat/6pt harness system before doing so.