I am a recreational diver. I would even go as far as to say that I am a vacation diver. Please allow me to address your questions.
First, with regards to why a rec diver would need a back-up computer, a couple years ago, I was on a LOB, and my computer failed. Luckily, the boat had a spare which they were kind enough to lend me for the rest of the week. If they didn't, then I would have had no computer and would have been done diving for the week. Having a Back-up or a redundancy is never a bad idea. When I got home, I bought a computer that I could use as a back up. As the saying goes, "Two is one, and one is zero."
Second, with regards to AI, I don't understand your issue with a recreational diver having AI. It just makes sense to have your information located on the same device. A diver can look at their AI computer and can check all relevant information. Doing it any other way is simply not efficient. Many (most???) AI computers also display both NDL and some variation of GTR, so the diver constantly knows whether their dive will be limited by NDL or by GTR and how long they have before they have to end the dive.
I don't imagine for a second that I have convinced you, but that is OK. We are both entitled to our opinions.