SuPrBuGmAn
Contributor
As SuPrBuGmAn has explained the redundancy and ability to isolate supply vs regulator gives you alot more options when "it hits the fan".
This is where the debates start. More options when the "shiot hits the fan" is quite subjective. Depending on the failure or problem, something catastrophic with a very quick loss of gas might end up depleting more gas than your reserve on a manifolded doubles setup while you work the isolator(or worse, work other valves first).
In an independant setup, you can only lose one tank. If you managed your gas correctly, you'll always have enough gas to exit on a single tank(incase of losing an entire tank at max penetration).
It all depends on the problem and how quickly you can react with it.
If I were diving backmount, I'd be diving with a manifold and isolator(infact thats what I did when I dove backmount). Presently I'm diving independants, in sidemount configuration and I think its safer in every respect.