Once gain, that's not entirely true and "maim" is way too strong a word. A Predator or Petrel will give you ample warning with red or flashing amber messages indicating that you are doing something stupid and it will in many cases require you to acknowledge that act of stupidity.
It does leave you firmly in charge however. For example if you end your first dive of the day on O2, the Predator will leave O2 as the selected gas, rather than defaulting to something else. The end result is that as you descend below 20 feet on your next dive, it will start giving you warning messages that you are on 02 and below 20' creating a PO2 above 1.6. Now, you are not really on O2, you just failed to re-set the computer to the proper gas at the beginning of the dive. And if you miss the warning for a minute or two on descent while you are busy with other things, a Predator or Petrel won't lock you out. It will happily let you change the computer to the right gas and let you continue the dive. The only harm is that it's been calculating the nitrogen loading and total CNS load based on 100% O2 for those few minutes of the dive. You should however be smart enough to understand what that means and perhaps pad your final deco stop by a minute or two to account for that.
That's preferable to the computer defaulting to some other programmed gas that may or may not be present. For example, some dive computers I have owned automatically default to air, which means I would have no warning of the gas selection error in the above example, and by the time I noticed the problem, the tissue loading information could be seriously in error, forcing a much longer deco than necessary, or reversion to tables. With the Predator or Petrel you only make the "forget to change the active gas to something other than O2" only once or twice before you add that item to your mental pre-dive checklist.
Another thing it assumes is that all the programmed gasses that are "on" will be used in the dive. That means the total time to surface will be shorter than actual if you don't in fact have all those gases available. Shearwater added the "on/off" function to the gas menu a few years ago as a response to customer input, and it was a nice change as prior to that you had to remove any other regularly used gasses from the menu to avoid having them used in the TTS calculation. However now you can leave them in the computer, and just turn them "off". It also means you can leave the deco gasses "off" until you need them, which means the TTS is actually your much more conservative lost deco gas time to surface. In short, you can run it however you want to run it, as long as you understand what it's doing, but that's not the same as maiming yourself.
To my knowledge the only time a Petrel or Predator will assume anything is when you bail out from CC to OC. It automatically selects the lowest PO2 gas suitable for the depth that is available in the computer settings, and then assumes you're on that gas, until you tell it other wise, as Shearwater assumes you might be task loaded with other issues for a minute or so after bailing out.
---------- Post added November 27th, 2015 at 11:35 AM ----------
I don't disagree.
Few people actually put in 100 dives a year. Freshwater divers or shore divers who are not tied to an expensive dive charter that gets blown out half the time will have a much better chance of doing 100 dives a year. Similarly, if you own your own boat and slip it at a marina you'll have a lot more opportunities to dive.
On the other hand, Marci got OW certified and on her first ever post certification dive decided she wanted to be a cave diver. About a year later at dive 100 she was in Cavern class, and after Cavern and Intro she drove from NC to FL every other weekend to cave dive.
Now, we make an average of 4 trips a year to FL or MX with usually 7 diving days per trip, which means in a normal year we can get 56 cave dives per year, each averaging about 100-120 minutes.
That however is also not the norm, as we've been cave divers for over 9 years now. The retention rate after 5 years of cave diving is down around 20%. The vast majority of cave divers, in in particular non local zip code cave divers, get into cave diving and only hang around a few years before dropping out.
The point here is that if you are driven and know exactly what you want to do, you should just get the right equipment to start with. I suspect one reason some cavern and intro to cave divers drop out is due to the cost of re-equipping with gear that is well suited to safe and comfortable cave diving at the full cave level. On the other hand you'll have no problem doing open water pretty fish dives with a subset of your full cave diving equipment.
