Many people don't have a convenient studio, and even if they do the interesting person they want to photograph probably wouldn't want to relocate... 'Just as well shoot indoor' is frequently not an option!Why would anyone use ND for indoor portraits?
Just like why would you use f 2.8, for portraits? do you like half of the face out of focus?
I have never truly grasped the concept of tight dof with location portraits, then you can just as well shoot indoor. It’s actually a good thing that you can see the background as it benefits to the story of the picture, why on earth would you want it to blurry out.
If the background adds to the image it doesn't make much sense to 'blurry it out' but often it's a distraction and often when it does add it becomes distracting if fully sharp, a slightly blurred background can hint at the location enough to provide the story. Like so much of photography a balance is required!
I did not write that it should be sharp or in focus, I where referring to the portraits where you can’t see what the background is. One of the points of shooting on location is that it adds to the story, if you remove that it’s as good as shooting in a studio without the benefit of being in one.
I have yet to experience where I’m not able to find a decent spot even if the subject where reluctant to travel.
Who say you need a studio! there are very skilled photographers who never use one, as they only do location. For me it’s a bit of a lazy trick, that doesn’t do any subject right. A background dos not need to be perfect, small imperfections can be photoshopped or managed with different angles/perspectives.