I think that in many cases they are. If the "normal" diver is one who goes down to the islands for a week of diving and does two tanks each morning or afternoon flowing a guide about, then a computer is by-and-large irrelevant.
I think that for the two tank per day for a week diver, owning a computer is possibly overkill. All the Hawaii dive boats I have been crew on have computers available for every guest, some even insist. For tables divers the boat I currently crew will usually restrict your bottom time on the 2nd dive (compared to the computer divers), and that disclosure during check-in nearly always results in the $5 computer rental.
