Plateau horns are mostly disliked by clarinet players, but for young girls with slender fingers, people with physical defects, and sloppy sax players who are learning the horn on their own, they are a Godsend. I think that most of the perception of the effect on tone is to the player rather than to the listener (at least in my experience), but I've never seen it scientifically tested.
I don't like them myself, but that's just me. This may partially be due to the only ones commonly available being from the LeBlanc lines.
The first two reasons have some real validity for ownership, the second not so much so. In the end, it's what works that counts.