1) Computers can and do fail...my DELL desktop's hard drive died in less than 1 year after I bought it. Imagine your comp dying just before your deco stop.
There's a world of difference between a PC and a dive computer. PC's crash because you have one operating system trying to run on a nearly infinite number of hardware configurations. Dive computers don't suffer from this problem. You also have to take into account that a hard drive has all sorts of moving parts that have to be in just the exact right place at the right time or you will experience failure. A single human hair is an enormous obstacle in regards to the distance between a read head and the platter in a hard drive. A dive computer is all solid state, so doesn't suffer from those problems either.
That's not to say that dive computers don't fail. Of course they do. But comparing them to your home PC is a useless argument for the reasons I listed above. ANY timing device can fail. You don't honestly think that your digital bottom timer isn't a computer, do you? It's just doing one or two less calculations (deco \ o2 exposure) than your bottom timer.
Potential failure is a really lousy reason to argue against a computer. Ask any of the bottom timer divers around here what they do if their bottom timer fails. Chances are their response will be something along the lines of "I pay attention to my dive profile". They keep a running image of their profile in their head while the dive progresses. If the bottom timer fails, then they can resort to their own internal timer to make a safe exit.
The same could be done with a dive computer. UP himself uses a dive computer. It's just in guage mode. Surely if dive computers had a larger failure rate than bottom timers, he wouldn't touch that thing, would he?
The argument against dive computers really doesn't have anything to do with failure. It's that they encourage you to stop thinking. You stop keeping track of the dive because the computer is doing it for you. It keeps track of your deco obligation and ascent rate. It encourages complacency. That's not to say you WILL become complacent or that you WILL stop paying attention while using a computer, but it's certainly easier to do than if you've been diligent about tracking your dives on your own.
It's only when you rely 100% on the computer that a failure can be a problem. If I were using a bottom timer but didn't look at it regularly, so I didn't have any idea of my dive time or my depth, than a bottom timer failure could be just as disastrous.