I remember when I first went to the dojo, wondering why these people were so noisy about hitting the floor and, thinking I will never slap the mat because I don't like making a lot of noise when I move. But after a while I found myself actually doing so when I am taking particularly fast ukemi and that it does seem to help dissipate large amounts of energy. And sometimes it's just plain fun.
Of course a lot of the time loud slaps are just not necessary and as has been pointed out sometimes it's not desirable to slap even when taking a fast fall.
So... I practice both and I practice being mindful enough to decide when and whether to slap the mat.