Different strokes for different folks. Weak swimmers who enjoy SCUBA are well-advised to keep the reg in as stated by many above. On the other hand, others including (but not limited to) experienced surfers, freedivers, competitive swimmers and triathletes, have a very different relationship with the water and might reasonably decide that they'd rather chat, save gas by using their snorkel, or whatever.
I can't imagine telling a surfer, who routinely gets pinned to the bottom by rough surf after wipeouts, that he might drown if he doesn't keep his reg in. I'd be laughed off the boat!