It's not about CMYK though - that's an even smaller gamut than RGB.
That depends on the RGB colourspace surely? But also...
For a photo editing program, you need a color space that is as large or larger than human vision.
Well you only need a gamut as large as the best device you are intending to display the image on.
Yeah like I said a small number of people really into photos need this.
Editing photos directly in RGB is the equivalent of a video production company (back in analog days) creating and editing videos exclusively on VHS tape. There's information loss every time you make a generation copy.
No, you don't generation loss in the same way. You may get loss entering the colourspace, IF any pixels saturate in either direction (i.e. are not representable in the new gamut), but you won't get any loss within the colourspace from the colourspace itself simply by doing stuff there. You might get quantization loss, of course, so you may well want more bits to avoid those accumulating.
But also my point stands: almost no one knows about that stuff, and based on the quality of stuff I see around and about a lot of professional stuff barely exceeds what you can do in MS Paint.