The point is that quantizing the range makes it easier for humans to choose colors. But there's already the #ABC hex format, which while less intuitive to non-techies has the huge advantage of being well-established.
But it doesn't make it easier for humans to choose colors. For a specific list of detent colors, it reduces the amount you have to memorize relative to full RGB. But to actually reason about colors, you want a non-arbitrary scale; HSV (for instance) gives you hue direction and then you can slide saturation and brightness around.
I don’t know, but I use #ABC a lot, it’s much more convenient than #ABCDEF, never mind [0, 256) or [0, 1]. There are of course more intuitive coordinate schemes and color models, but I find RGB easy enough when you’re not actually doing serious graphic design. This is not about having a GUI color picker either, this is about hand-typing colors.
Maybe it’s just because I’m old and wrote CSS way before it got HSL or other fancy color functions, but personally, RGB colors are really deeply entrenched in my brain.
I think my thing here is, you can do any notation for colors you want. "Splash" is custom. So you might as well do a better custom. "rrb85" for "red, red, blue, 80% sat, 50% value" for a dark purple --- one step towards red from the midpoint between red and blue. I don't know, something! RGB is kind of bad!
Despite my background in color science, I find RGB more intuitive. With HSV I have to remember the chirality of hue and it's zero point, and when changing hue I find it difficult to reason about saturation. In practice this means I must "nudge and judge" with both systems. With RGB I can always make progress. With HSV I guess hue wrong about half the time. I could probably improve this.
To be fair, I'm also colorblind. That's probably relevant.
Anyway, I'd say the answer to both your questions is: "sometimes"
All you're arguing is that it's easier to memorize primary and simple secondary colors in RGB. No question, it is. Once you've got one of those detent colors locked in, how do you vary it? What does it mean to bring up i% of the first channel, j% of the second, and k% of the third? That's the problem HSV solves.