Whether it is necessary or not isn't the question. It would certainly be fun to have!
Of course it is. Developing support for 2nd monitor display inevitably takes development time off something else, so it's very much a question of necessity. It might be "handy" to have in a few cases, but none of those are compelling enough to say "Yes, we need dev time on this feature".
Well, at no point did I think they would be able to just make this pop into existence before the game went gold. So that negates the development time related issue if it is something that is a quality of life/gameplay enhancement. This is a general related "Wish List" wouldn't it be nice to have and utilize.
Something that you don't get with consoles. Something that is PC specific. Again that said, depending on the actual difficulty to make this happen would be entirely up for them to decide. They are the professionals and know what it would take to achieve this or that result.
But I'm going to disagree in general with your premise that in the future we should ignore this idea due to a perceived value being taken away from other areas of enhancement. My predication though is simply based upon interest, and for them to determine feasibility compared to value and quality of game enhancements.
Hence the suggestions, for future ideas, even if they are not always, "ideal". More along the lines of, utilizing a feature that in this modern day, is not far fetched for people having multiple monitors.