I admit I have not read through the entire thread but I run a dive center so here is my two bar:
Many dive centers rent dive computers. They will tell you the cost of the dives, say "two dives with complete gear" but then tell you a computer is extra. If the computer is extra, they are in fact, not renting you full gear.
At my dive center, we decided that full gear meant just that, full gear and a dive computer is included. Relying on the instructor or dive guide is dangerous and should never be done. It is taught in the open water course to never share a computer or rely on your buddies computer but we see dive centers around the world do this all the time.
I tell all my new divers the first two pieces of gear they should buy is a good fitting mask and a computer. Knowing what that computer is telling you and where to find the information you need when on a dive is key which is why constantly renting different computers is suboptimal at best.