Fell hard on concrete while washing my car a while back (soap water on painted concrete surface=very slippery). Fell backwards, heels over head (like a Tom and Jerry Cartoon), instinctively extended arms outward (handblade downwards), hood of car got in the way of my hand on the way down. Did not break my neck or crack the back of my head (which would have been very probable without ukemi training the way my body was oriented), the hood of the car still shows the dent left by my hand, besides a light stinging the hand was fine. All in all it was fun, I got up laughing and marvelling at how ukemi works. The "slap" did save me from getting a concussion or worse (since my head would have taken the entire impact otherwise). I believe in that situation slapping or "extending" was critical to regain some semblance of balance before my head and torso hit and to distribute the force away from critical areas of my body that were not so susceptible to impact. But I also know that I would not be trying that sort of fall on a jagged, uneven surface covered in glass as well. It depends on the environment imho.
Regarding rolls though I can see how slapping is entirely unnecessary.