My uke for my nikyu test did that. The testing result was that I looked like a total badass because it appeared I was slamming him. There is totally a fun factor in having an uke that hits so hard it makes you look awesome. That being said, I'm pretty sure the net result is going to be you winding up in a lot of pain later on down the road. I won't say that slamming the mat full force with your slapping hand should never happen, but be aware of it and only do it at times when you want that extra effect, imo.
Yeah, I see that a lot. I don't like it because it takes uke way too long to get back up. Whereas if you can land controlled you can start attacking again as you get back up (i.e. that clip I put in just now).
Another thing that is my pet peeve; uke slamming around and ones that grunt and moan when getting thrown with some energy or have a solid lock against them. I feel like this actually takes away from the person doing the technique. If I'm taking technique from my teacher, I try to be as unnoticeable as possible. I do ukemi to learn, and help others teach or learn. If I'm slapping the mat, grunting in pain, etc, I feel like that's selfish and puts attention on the uke vice the person doing technique.