Geezuz, you guys are only diving, not going to the Moon...why two computers, waste of money, go on a vacation instead
I used to believe as you did. But:
1). Having had my (old) single dive computer die on my on no less than three occasions - yes, during a dive - I don't intend to be in that situation again. The first time it happened, I replaced the computer (with the same model), and blow me down if it didn't happen again. Fortunately, I always carry dive tables with me - they're in my log book and travel with me on every vacation. I had no problem cutting tables for subsequent dives (a kindly DM/fellow traveler lent me depth gauges and bottom-timers), but in the name of conservatism, I had to skip a couple of dives during the rest of my vacations. Sod that, and never again!
2). Having started down cave diving path, I like the added security and redundancy of carrying two computers.
3). I'll eventually take an Advanced Nitrox/Decompression Proceedings course, and since my Oceanic's not a full decompression computer and handles O2 mixes of up to 50%, it's not really equipped to continue serving as my primary computer. Using my Dive Rite now is great practice, and moreover, allows me the comfort and familiarity of using my old friend, the Oceanic, too.
4). Having a diver as a partner = double the toys. Especially because he's upgraded to a rebreather. Some of his older computers, like his Dive Rite, pale in comparison to the Shearwater he now flies. All the more for me, I say!