You could do the same thing manually, using your mechanical depth gauge and bottom timer and tables that you take with you. The computer provides convenience, increased accuracy, and eliminates the possibility of a possibly-narced human making an arithmetic error.
No, you get way longer rec dives with a computer than what you can plan with tables... the difference is huge.
Cave or deko dives need to be planned out anyways, so you don't need that feature.
I have used AI on at least 300 or 400 dives, after it broke I bought a normal rec computer. I never once used the 'rest air time'. You should know your sac before you do any kind of 'advanced' diving. For very shallow dives you don't need it either because you can just dive untill you hit your turn pressure or back to the boat pressure.
As I said before, if you like AI use it, it's not gonna kill you, but don't tell yourself it's gonna help you.
With an SPG on your waist, glancing at your computer is not going to clue you in that you're sucking down your gas faster than planned.
What? Unless your computer tells you that you have been breathing more you wouldn't notice it? You're the one who's breathing. If this really is the case you need to pay more attention. Don't rely on the computer to do your thinking. There is no excuse for getting distracted for more than a few minutes at a time.
You should always know how much gas you have left and how long it's gonna last you, you just look at you SPG to check whether you are right ot not.
Edit: Technically an SPG is more likely to leak, BUT you lose a very small amout of gas when it leaks, so it's not really an issue. On many dive trips you will see people with leaking SPGs and some don't even notice it or don't care.