> Does it tell you whether there are highlights or shadows that can be recovered with Adobe Camera Raw, for example?
Yes, among other things.
> how often is that a key factor in the workflow of taking a bunch of images from a shoot, culling and processing them?
When bracketing a high dynamic range scene, it is often a key factor. Correct exposure is considered in photography to be one of key factors, no?
> Why would looking at the out of camera JPEG not be a better starting point for triage?
Because JPEG has nearly nothing to do with the raw data. It will show you wrong brightness, and can easily show red or blue channels being blown out when in fact they are underexposed.
> OK, so what practical problem does having an accurate histogram solve?
May I ask why are you willing to concentrate on just one thing, histogram?