ETTR results in more noticeable glare in real world conditions. Modern cameras are fairly clean noise wise, even at fairly high ISO it's inoffensive, glare is more of a problem in reducing detail than noise is. Thus, the photographer will want to correct for glare that might be linear (veiling global glare), or non-linear (localized) in post. This all takes time.
I agree that global veiling glare (very low spatial frequency due to outside-of-FOV) and more localized glare/flare (due to bright light sources, or bright reflective surfaces, within the FOV) can be a pain. However, I'm undecided about it's non-linearity in its contribution to the final exposure. Given an amount of flare for a given scene with a given optical path and exposure level, only increasing the exposure level
would result in a linear contribution of all elements that add to the exposure, assuming that the sensor has a linear response (which CCDs an CMOS devices do in general, save for provisions for draining overexposure), or so it would seem.
Yet John McCann also suggests
that the veiling glare contribution is non-linear. I can imagine that being the case for intra-occular glare, because the change of the pupil diameter of our eyes is a function of the brightness, and thus of the optical and thus glare performance. However, when only the exposure time is a variable, I sofar fail to see the non-linear contributing component(s).
I shoot a lot of architecture and glare is a frequent unwelcome guest. Here's a very interesting paper on (rather extreme) glare and various solutions: http://graphics.stanford.edu/papers/glare_removal/
Indeed, architecture is a more obvious scenario in which I also encounter the detrimental effects of veiling glare (which can be minimized on a given lens by using a good adjustable
lens hood, and in general by using lenses with few lens-groups, good coating, and proper internal lens edge and barrel blackening). Some lenses even use multiple apertures to suppress internal reflections.
The only reason for non-linearity that I can identify sofar, is that it's spatially variant (i.e. subject to local brightness), but I don't see the non-linear aspect of a uniform change in exposure level (by varying exposure time, not by changing aperture (which is a no-no in HDR photography)) for a given scene.
Given John McCann's examples, I feel that I'm overlooking something obvious ..., but sofar I'm unable to pin-point it.