Well, just because a control is there doesn't mean you are under any obligation to use it...as long as you DO know what it does, if it doesn't do anything you want, don't use it...
Of course. I completely agree with you. But two comments:
1) It appears that the negative values for clarity may have been provided as a way to make the interface consistent with all the sliders being in the middle zero value and being able to go both ways (negative and positive). I believe in previous generations some controls started with zero on the far left and only slid to positive values. Of course, some people are saying they have good results using it for portrait work, so who am I to say it is worthless? I'm not loving it. To me, negative clarity isn't really an amazing control like say shadows, highlights, or CA adjustments, or noise reduction, or vibrance, or blah blah blah.
...but with that said...
2) There are lots of controls in PS that I initially can't find a use for in my workflow, but eventually I am able to make useful in seemingly weird and unconventional ways. Sometimes it takes someone showing me how they use it to spark my creativity to see how I can use it. I am not yet seeing that for negative clarity in landscape work, and that is the whole point of my query. Can someone show me something great with negative clarity?
For each image, I can't possibly try every permutation of all the adjustments I can make. I need to have some starting points, and ruling out negative clarity, negative whites, and positive blacks has helped me focus where I think I can make the greatest value. Of course, there are probably some corner cases where these things are useful, but I can't figure out what they are.
Let me think: positive blacks......hmmm.....low contrast, gray, noisy, blocky posterization. What could I use that for. I guess I could use it to simulate duotone printing on newsprint. Yuck. Not for me.
Let me see: negative whites.... hmmm...more useful because the control doesn't so severely display the limits of digital capture with noise and posterization. This is probably useful for working on something fogy and low key. This would possibly be interesting if I were trying to emulate some of the early (19th century) photographers work that has faded over the years. It reminds me of some of the instagram filters that give the image a cyan or magenta cast decreases contrast and makes it look like a 1970s Instamatic image. More legitimate than positive blacks, undoubtedly, but still kind of a "screw-it-up" adjustment. I could probably get a better effect with negative contrast.
I freely admit this is all stylistic. I don't like adjustments that impart attributes that diminish image quality in a way that seeks to emulate inferior or nostalgic photographic performance. Holga cameras aren't my cup of tea and neither are Holga filters. Again this is personal, but photo effects that call too much attention to themselves are poor effects.
This is not to say I don't like moody or dramatic interpretations. I just can't see these controls (lighten blacks, darken whites in particular) being very useful.