Interesting so far, I am surprised that some form of redundancy outweighs none by 61 to 39% at this time.
To address some of the deciding factors mentioned here, my own diving is local double tank boat diving and shore diving, every now and then a trip up to the GBR involving a few days of diving on day boats, have not been on a proper liveaboard for 10 years now because my lady is a non-diver and living right next to the ocean, most extended travel goes to non-beach destinations.
I think I feel fine diving without a backup locally as I can handle sitting out a second dive, but will acquire a backup for the next significant dive trip if no wrist-mounted computers can be rented as a backup. I may have to resort to screwing a spare SPG, which should be found on any of the larger dive boats, into my reg for a subsequent dive so as not to invest in a second AI-capable computer and backup transmitter to cover all potential failure points.
I used to wear dive watches, the capable and expensive kind, ironically in a non-diving phase and thus they have never seen SCUBA action. Now that I own only one non-diver watch for the benefit of daily wear, I won't take it diving due not having a dive bezel and so as not to deteriorate the seals with a lot of salt water exposure, but I sure wish I did not sell the divers in hindsight. Fact is, unless they have a depth complication via bourdon tubes or membranes (IWC Aquatimer Deep Three comes to mind) to be worked with using max depth and assuming a square profile on tables for the next dive, they are of limited use as a backup. The problem with most of these watches is that you can stack a bunch of Terics on each limb and have a drawer full of transmitters for the price of otherwise fairly unattractive (to me) watches.
I think the biggest benefit of a dive watch these days, beyond the 'quick glance' appreciation of time, is using the bezel not to look at total dive time but to estimate distance travelled for nav purposes.
I would leave the snorkel at home if I could (required to have one on you by law here, although it never saw a mask strap) but I am a believer in carrying a spare mask on every dive. Short of the octopus, this is the only redundancy I currently have on me underwater.