However, as others have said, it's a judgment call in each particular situation. Except in extremely clean drops, standing in the rockfall zone is a very bad idea. But usually it is possible to stand sufficiently to the side or, more often, under sufficient cover to make the risk of being hit by a rock very small indeed. A more serious problem with bottom belaying is that, for a long enough drop, it tends not to be effective. And it should not be considered a sure thing to work, even for a shorter drop.
I think bottom belays are more important for beginners but I think it's not limited to them. Sometimes someone may not want a bottom belay for various reasons, but unless someone expresses their desire not to have one (and you know they are sufficiently experienced to make that call), I think one should weigh the various factors and make a decision. For experienced cavers, often time (progressing more quickly through the cave) is a good reason not to bother with them. But every time I am on a trip, I think about whether or not bottom belaying would make sense, and if I think it would, I offer (or request) it. Occasionally I think a bottom belay might be a good idea, and an experienced caver refuses it. But I've never had anyone object to my bringing up the question.
Billy wrote:Not to further quibble/nitpick, but this guy is not in a climbing situation, he is canyoneering. He is also doing a single rope (instead of a doubled rope) rap. Climbers almost always (actually I don't know anyone, but I'm sure someone will correct me on on a new burgeoning style or area where it is done) double rope rap.
Rappelling on a doubled rope is a nice way to get your rope down afterwards, and to avoid having to reconfigure it too much after (or occasionally, as in toproping, before) climbing. But doubling a rope is not primarily done as a safety measure. It does increase friction, but you might use your device somewhat differently to compensate, and if you let go of just one side (or it slipped or broke), the rope would run through the top and come down. While rock and ice climbing, I (and other climbers including those far more experienced) have rappelled on single ropes, though this is most often done with a dedicated rappel line that is not also climbed on.