Yes, it is a good idea to keep your regulator in your mouth from jump to return to boat or if shore diving until completion of the dive though some may use a snorkel for shore diving swims.
But here is why, you just completed a near hour dive, saw all sorts of cool things, everyone is amped up and the boat is far away picking up the air hog group and we want to talk and kid around. And some folks grew up in the water and a little saltwater down the pipe or in the eyes or face just is not going to set me or them to a teether. Lay back on my wing, breathe the fresh ocean air and chat about our adventure. That is why. Oh, and to complete my ensemble, I might even put my mask on my forehead where in some 56 years as an active diver I have never lost a mask. And on the subject of these all inclusive always rules, I do not ever take my fins off until I have a foot on the ladder and my hands on the ladder.
Has anyone ever managed to body surf in full kit on a wave to shore? And then got face planted? How can I yell kawabunga or whatever it is surfers yell nowadays with a regulator in my mouth, splat, that hurt!