CMYK exists because, though is possible theoretically, it isn't possible
to generate black from mixing CMY inks. As the C, M and Y inks aren't
perfect and have some contaminants, mixing them ends up in a dirty brown
instead of pure black.
That's why CMYK exists, and that's why it isn't so simple to print an
RGB image.
The problem resides mostly in the generation of black and gray shades.
Although there are systems that do a great job converting a photo to
CMYK on the print side, it's still a problem to do a simple task as
placing a pure black overprint on a solid color background without
destroying some underlaying information on the separation.
I'm a designer, not a photographer, and an image manipulation program is
an essential part of my workflow. And placing some black text, or
editing a large image with a black or gray background, adding black drop
shadows, aren't rare at all. And it's a pain without the ability of
editing the separated CMYK.
It's not about if the printer will handle the file or not, is about
creative control. Sometimes you NEED to control the black overprint.
Sometimes you need to use spot colors and you need to control the
channels and how they overprint.
Even with Adobe software, before having spot channels in Photoshop, it
was a common practice to separate to CMYK to make 2, 3 or for 2 inks
prints (replacing the corresponding plate's ink for a custom ink).
Simply because other programs didn't support the Adobe's custom
multitone files but did support CMYK tiffs.

Well, I can't do duotones with Gimp to insert in a 2 inks flyer.
CMYK editing would help. I can't control the black generation of a
separation, because the separate+ plugin doesn't support that. It just
support existing profiles and there is no control. I can't control CMYK
curves. And trust me, that's extremely common.

I can live without CMYK, I have some workarounds and can do some tricks,
but it certainly makes my designer work harder and less productive.
I can wait, I'm ok with the argument "it's not trivial, requires a lot
of work, requires a lot of refactoring". But I'm not ok when somebody
says that it isn't necessary.
Maybe it isn't for photographers, but it certainly is for designers.
GIMP stands for GNU Image Manipulation Program. Not just for Photograpy.
I think that CMYK editing is certainly in the scope of the product

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