Liveaboards require them frequently. Here is a story that may help explain why.
The Mike Ball liveaboard operation runs liveaboards in Australia. In 2003 they had an advertised policy of requiring checkout dives, but they made an exception when a man named Gabe Watson said that his NASDS Rescue Diver certification indicated he did not need one, and he would be able to take care of his newly certified wife, Tina, as well. It turned out later that Gabe had about as little experience as you could have with that level of certification, and the thoroughness of his courses were very suspect. (His 4-day Rescue Diver class was completed in 2 days.) On their first dive, Tina was quite incompetent, and Gabe's attempts to rescue her was so very incompetent that some people believed he intentionally killed her, leading to years of legal issues and court cases. In the long run, experts determined that he was simply an incompetent, inexperienced diver, and neither he nor his wife should not have been allowed to do that dive unsupervised.
Mike Ball was heavily fined for not performing the checkout dive. There was no law requiring it--they were fined for failing to enforce their own policy.
I did a checkout dive when I did a liveaboard in Australia, and you can bet they made me do it, even though they knew I was an instructor. No problem. In fact, I'm glad we did it. All the customers were grouped by their perceived abilities on the checkout dive, with each group assigned a specific guide. My two diving companions and I had no required guide, and we could do whatever we wanted.