Q:

Why use UniWB? Doesn't apparent clipping of "final" colors matter more than actual clipping of RAW sensor values?

I've been reading up on UniWB and trying it out myself. My understanding is that the camera (and other RAW software?), in applying a white balance during the JPEG conversion, effectively overexposes the R and B channels (usually by a factor of around 2 to 2.5). The R and B channels may therefore appear clipped when in fact they are not saturated in the RAW data. Using UniWB white balance on the camera effectively sets all of these overexposure factors to 1, which means the histograms give a much more accurate representation of clipping of the RAW values.
However, one naturally never uses the UniWB white balance setting when post-processing. According to my understanding, applying a more sensible white balance in post may still clip the R and B channels in the result, just like in the camera. Thus, even though the R and B channels always are fully recoverable by drastically changing white balance or color settings (which may not be desirable), they may still appear clipped in the final result (appearing as so-called "nuclear colors").
My two questions:

If the above is true, wouldn't it be better to set a white balance in the camera as close as possible to what you will actually use in the final result, since this would yield a more accurate histogram for your final result? (I.e. colors that appear clipped on the camera, while possibly not truly saturated in the RAW data, will still appear clipped in the final result, so you might want to adjust your exposure.)
Applying a non-UniWB white balance in camera may show channels as clipped even though they are not in fact saturated (i.e. the histogram is overly conservative). Can this also work the other way -- i.e. can "normal" white balance settings in the camera effectively show channels as not clipped even when they are in fact clipped? (I don't think this is possible since by my understanding, this would require exposure factors below 1, which are never used, but it would be nice to have a confirmation of this.)

A:

The whole point of UNIWB is avoid clipping the RAW data, something which is pretty much irreversible.
You ask what the point of this is, since you obviously don't want to use the UNIWB setting in your final image, and hence the red and blue channels will be clipped when applying a standard white balance. However, during RAW conversion you can reduce the exposure of the entire image to prevent any clipping whilst still enjoying a neutral white balance and correct colours. 
The histogram on your camera is designed to warn you of exposure problems, it is not a tool to set the final output exposure level, that is a matter of taste and should be done by eye (on calibrated equipment).
Clipping in the final image is not as bad as clipping the original capture. You may decide that an amount of clipping is acceptable if you really want a certain exposure level. You may loose detail but that's the trade-off you have to make. But if you clip the RAW data and you decide you don't want that exposure level, then it's too late. 
I don't know enough about how the white balance is applied  during in camera RAW conversion to say for sure that the brightness of a channel is always increased, never decreased, however in general there is usually 1 stop more detail in the RAW file than in the camera JPEG. This means unless the "exposure factor" is less than 0.5, which seems unlikely, the camera histogram will never give you a false negative for RAW clipping.

Ultimately UNIWB is a tool for determining the maximum exposure possible (with the implicit assumption that you pull this back in post) which carries the principal benefit of lower noise (and slower shutter speeds/faster apertures if required). My advice in these situations is simply to bracket your exposures whenever you have the storage space to do so, that way almost any metering/clipping problems you might experience simply go away.

