Um in my part of the world, all the recreational certified divers usually follow the DM who has the dive computer. The rest of the divers either dont have a computer or dont plan their dives. And decompression related accidents are rare, maybe because the DMs dont push their profiles to the limit.
If you ask me, the idea behind different nitrogen loading etc etc since every diver will have a slightly different profile makes perfect sense. By that theory, every diver should have his own dive computer.
In reality (from my experience at least) while I wouldn't recommend it, group diving with only 1 DC has been done and will probably continue to be done recreationally in South East Asia. As long as the whole group sticks together and their profiles dont vary too much, there shouldnt be a problem, especially if the dives are on shallow reefs of less than 20m. You'll run out of air before you run out of time.
In theory, all dives go exactly as planned and all divers act exactly as you expect them to. In reality, Murphy and Darwin are co-conspirators. And nothing they do is personal.
Bending the "rules" a bit is rarely a good idea. Suggesting to new divers (the purpose of this forum) that it is ok for them to bend the rules is a really bad idea. And just to be clear: there is nothing wrong with diving tables or having one person dive tables while the other dives a computer (provided that the most conservative of the two governs your dive plan).
I do agree that the practice you suggest does happen with some divers in some places. Here are some reasons why it is a bad idea:
You should never hand over your safety responsibilities to another. If someone else is setting the dive plan, that is fine. But, IMHO, you still have a responsibility to verify that plan as being safe for you. (Yes, I have seen DM provided dive plans that would put my old USD computer into deco).
Dives don't always go as planned. Divers get separated. Some have trouble with bouyancy and might have a more seasaw profile. There can be downdrafts. Divers might ascend faster than the DM.
Small differences in profile can matter (depth, time, surface interval, ascent rates). Staying just a few feet deeper for a long period of time can have an impact -- especially on the second and third dives. Also: you need to be sure that any RBT carried from day to day (yes, I have had enough dives where this happens) stays on that same computer. Can you even guarantee that the exact same computer is going to be on the second dive of the day?
If something bad happens, you (not someone else) pays the price. Recreational diving is, IMHO, a very safe sport. But, if you get bent, it is you who deals with the chamber ride, the bills and the rehab.
