It was asked earlier why we keep the pressure so close on tanks, let me relate an experience from this last weekend.
I did two dives in Ginnie Springs this past weekend. The first dive was way cool and we penetrated about 1,000' back (almost to the Maple Leaf). I started with 3600 across both cylinders and when the dive was turned I was at 2600 and 2400 respectively. I did get a little sloppy with my switches on the way back which resulted in there being 450 psi difference at one point. I finished the dive with both cylinders within a 100 psi.
On the second dive we got back around 1100' on Hill 400. Since I started with just a bit under 3300 in each cylinder, my turn was 2200, We turned just as I reached 2200 on one with 2400 in the other. I had no problem with keeping within 300 during the dive, and on the way out I was treated to the "Eye Bypass" which is a squiggly little bypass that you have to think and slink your way through. It was fun and I believe I was within 200 psi at the end of the dive.
On the bottom of the steps, the three of us were unhooking our bottles and such when my left first stage let loose. WHHHHHHHHHHHHHOOOOSHHHHHHHHHHHHHHHHH. The HP seat/valve had failed and I quickly turned off the air. As one of my buddies commented: "Talk about perfect timing for a failure!" Wow. What if that had happened while I was still in there? Holy Schnieke! I would have been left with only one tank to breathe off of.
Now imagine if I had only drained the right tank (used 2/3s of it or 1/3 total) and this happened when I was 1100' back? If I was diving solo I might have died. Yes, it takes a lot less gas to exit Ginnie then to penetrate, but I would have only half the gas that I came in on. Even with a buddy, I would have severely limited my options as to how I would exit. Keeping the tanks within a few hundred PSI maintains my redundancy. That's a good thing.