I'm guessing that Stefano was actually referring to people who "tap" the mat rather than "slap" the mat, but I'll give my thoughts on both, just in case.
I think that tapping the mat gives the person a way to time their roll. Over time, we all develop our own ukemi "styles" and approaches, and it sometimes helps to have these kinds of timing cues.
As far as slapping during a roll goes, I'm one of those folks who tends to slap pretty loudly during back falls to take some of the impact off my body. However, I've been beginning to take the slap out of my forward rolls these days so that I can use the forward momentum to bring myself back up to a standing position.
Another reason why I've seen people slap loudly on the mat is to make the person training near them aware that there's someone on the ground there. I've deliberately slapped loudly on the mat when it wasn't "necessary" to let someone who wasn't looking very carefully know that I was thrown there...
As far as the silent/soft breakfalls go, I'm all for them. They're a great set of tools to learn, even if just to get a different set of movements in your body. I can't say I do them 100% of the time (as we really don't do that many high/breakfalls in our dojo), I do find myself doing them at times.