Yes, I have my time and depth without the computer, ascent rate irrevelant if managed appropriately, N2 status is derived pre-dive and while useful, not required during dive. A total computer failure would likely end the dive. I have yet to have a computer failure but have had transmitter failures, so I would say that it is more likely to have a transmitter failure than a computer failure. I have yet to have a SPG failure. So, while many rely exclusively on their computer, many have learned how to manage without one if necessary. Those who rely on only a computer should carry two. I choose to carry one and have other means to safely complete my dives. You may have never seen your transmitter fail and hence rely on it, I have seen mine fail and will carry and SPG. Different experiences, different choices, the nice thing about it is we can make our own choices, doesn't mean one is wrong or better than another.