Well, since our vision has low acuity under low light conditions, it is, from a photographic point of view, an open question whether the greatest possible sharpness is desirable for low light images.
I find a key factor in my interest in photography to be able to record something that our naked eyes can't quite see clearly. Anyway, this is such a common situation in landscape photography (for landscape, acceptable light quality often comes with penalty in brightness) that it would be quite limiting to not have some kind of facility to avoid mirror and shutter shake. If you don't need it you don't need it but where I live, those speeds are normal or a bit on the fast side (many now seem to use filters to get the exposure time to seconds, but I don't like the effect much, so I'll stick to my 1/25s thank you very much). "More often than not" rather than something unusual.
Leaving that aside, the technical question is where vibration caused by mirror motion ranks on the list of things that might adversely affect an image taken with a 300mm lens at 1/25 sec: atmospheric conditions, your finger on the shutter, vibration from passing trucks, the fact that you have an A9 and the only 300mm lens for it is a slow 70-300 zoom, etc. The answer is "way down", but if everything else has been controlled and you feel you need to eliminate the effect of mirror motion, fine: the camera is, obviously, on a tripod so you use mirror lock-up if you have it or shutter delay if you don't.
I don't find that to be correct. When photographing nature, one is typically not affected by passing trucks. Atmospherics can have an effect but all effects which reduce image sharpness are cumulative and I want to eliminate those that I can. The subject may not be so very far away, it could be a rock some 10 meters from shore, with beautiful ice formations that glows nicely in the pre-sunrise light. So in this distance atmospheric degradation isn't such a problem. Yes, almost all my shots that are of static subjects are taken on tripod, even if I can use a fast shutter speed for landscape (rare), since I want to precisely control the composition and use the lowest possible ISO, and stop down the lens. With a tele, hand held shots tend to vary greatly in composition from shot to shot due to the difficulty of holding it precisely fixed. So a tripod solves this problem and many others.
The question is whether cameras that don't have a mirror have an advantage because they never have mirror slap. The answer is that they do not, because in any circumstances when mirror slap might have an effect - usual or unusual for an individual as those circumstances may be - it is trivially easy to eliminate it.
It is easy to avoid it but there are some hoops one must go through to avoid mirror and shutter shake in Nikons. In the D810, EFCS is tied to the M-UP release mode and to get precise timing and avoid vibrations you have to use a remote release with it, and for a moment between the two button presses required, you lose both optical view and the LCD LV of the subject, so timing is based on viewing past the camera with one's eyes. Setting these things up can take a bit of time. For landscape and close-ups, I don't mind going through the hoops but if one is working on living subjects, and want to take advantage of vibration free shooting, there can be an issue. In the D5, LV + M-UP + EFCS can be combined behind a single press of the remote release button, though, but I don't know if it is genuinely delay free. Anyway, Nikon is introducing electronic shutter gradually in their cameras it seems and they're being very cautious about it. There can be rolling shutter effects and uneven exposure in some cases (with PC lenses when movements are used with fast shutter speed) and this may be why Nikon doesn't allow access to the electronic shutter without the hoops. They probably don't want us using it by accident and getting unexpected artifacts. I guess in future cameras we can expect easier to use variants of this feature.
Some users do claim that mirror and shutter vibration shooting does result in sharper hand held captures as well, e.g. dpreview staff wrote an article about it and they indicated that they were able to get better sharpness hand holding the D810 in M-UP mode with EFCS and just pressing twice than in normal shooting mode but of course there is the viewfinder blackout that makes this mode of operation impractical in that camera. Personally I am fine with the limitations of my DSLR setups (including the hoops one must pass through sometimes) and simply prefer to use a wide aperture and fast shutter speed to get a higher confidence of avoiding subject movement effects as well as camera shake - when the subject is of such a nature that it can move. And I love the optical viewfinder for photographing subjects that can, and do move.
