Why does everybody hate on RGB? If done right (a la Duck Lightsaver)with well diffused underglow and paired with thick non shine thru caps like GMK or SP it can really tie together a build. Plus you can always set the color to a static one or just turn it off completely.
I think it's being hated on because the functionality that's being marketed for it (flashy, sometimes reactive effects) are tacky and serve no practical purpose. But, I absolutely agree with you that it offers a key advantage by allowing you to choose the exact colour you want for static underglow/backlighting without swapping out LEDs.
And some effects can actually be useful in my eyes, especially when you're working with multiple layers. Changing the colour depending on the activated layer can be quite useful and IMO looks better than having indicator LEDs.
What's unnecessary is separate colour change for every single LED, having all LEDs show the same colour would suffice completely, but as addressable LEDs like the WS2812 are not that expensive and don't require additional driving circuitry, they're actually easier to implement.