Okay, it's a boring Sunday, so I'll give this one a shot.
Many liveaboards require a dive computer (we do not) for many reasons, but none of them are to make you safer. Scuba diving is not a safe sport, and a computer does not make it so.
With that said, I want to know your real profile. Since many are incompetent to use tables (or don't care to), I want to know how badly or how far you're in deco. That way, when I evacuate you to a chamber, I have an answer for the flight surgeon besides "Duuhhhh, I don't know". Whether you know your profile or not is irrelavant to me. I want to know what it is. And since if you're not responsible enough to wear a computer, you're probably not responsible enough to re-zero the little pointer on your rented depth gauge or bring a watch that can be used underwater.
Second, I want you to have the best experience possible. If you choose to dive tables, you will be bottom-time and depth limited on your second dive of the trip. When your buddies are hanging out with the manta at the end of the dive and you have to return to the boat (with a buddy) instead of hanging out with a manta, you're going to be pissed at me. And if you choose to hang out with the manta and violate tables, I'm going to sit you out for 24 hours, or until you're a clean diver again. That's going to piss you off too, then you will bitch and whine about what an ******* I am, and how you're never coming back, and you hope the boat sinks, and generally causing a sour attitude on the boat. I don't need that, and neither do the other passengers on the boat.
Third, I think training agencies suck. The last decent training agency was NASDS, and they couldn't compete with the 2 day course that other training agencies promote. The day some training agency tries to regulate the diving I allow on the Spree will be the day I give it back to the bank.
Frank