Hey, David, the older we get, the more set we become in our ways. Unless, of course, the Big Bird of Truth comes along and craps on your head! That happened to me, and I'm still wiping it out of my eyes and ears.
I came up slapping the mats like everyone else in aikido. Then, one day, I fell on ice and hard-slapped the ground like I always had done on the mats. The stinging in my arms kinda took my mind off the rest of the fall, so maybe that distraction is an actual benefit of slapping. Some months later, I was thrown off my horse when a shotgun went off, and while my head-tuck kept me from getting my brains bashed out on the boulders, my arms were badly bruised from hard slapping the ground. Thereafter, in class and in the "real world," I stopped slapping, and found that the relaxed extension of my arms spread out the impact of the fall over a greater surface area, with absolutely no harm or pain to my arms.
Years later, in discussing it with jujutsuka and judo guys, they recounted to me their instructors' exhortations to use the hard slap as, simply, an exercise to instill in them the habit of extending out their arms in a breakfall. In other words, the slap was intended to be "training wheels," not the actual real-life application of the breakfall technique. With time, they were to stop doing the slap and to remember to just open and extend the arms.
Just as extending your arms in the water increases the surface area of your body on the water, making you more bouyant, doing so when falling on the ground spreads and diffuses your body weight and the force of falling, over a larger area so no one part of your body takes the full brunt of the impact.
Remember those old saws that if you keep making a face, it will freeze that way? Or if you go swimming right after eating, you'll have cramps? Actively hard-slapping the ground to breakfall falls into that category of beliefs that are hard to let go of.