My local library has a REALLY old computer tucked away in the back, which I wanted to check out. I asked the librarians if it was ok, and after a minute of them staring at me like i was crazy, they agreed to let me have a look. Under closer inspection, I saw that the "keyboard" was just some kind of non standard layout numpad with some sort of mechanical keyswitch inside. After prying off one of the keys, I found to my surprise that underneath its ugly white exterior, the numpad held cherry clears!
I was really excited at first, since I wasn't sure if any real keyboard actually used them. So I typed for a bit and to my dismay, the typing experience was kind of bad. I had attacked them with the same mentality i had when i had my compaq with cherry browns, but found that the tactility was very lacking. While I've never actually tried a keyboard with mx blacks (besides the ones that they have at the best buy counters) , I can imagine that this is what they feel like, since the tactility in the clears was almost nonexistent. I can assume that the reason that they are not more popular is that they not only provide a mediocre typing experience, but they also are not fit for POS environments.