It helps people build an intuition. It doesn't need to be exact to do that.
It doesn't have to be exact, but it's counter productive when it is clearly and meaningfully incorrect though. That's the problem with the two dimensional [0,1] scale as well.
>so obviously incorrect though
I couldn't possibly disagree more.
That's just the limits of it being a metaphor. Audio mixers also only have a finite number of channels, but are also much less familiar to most people.
"spectrum" encompasses any hue, not just those 3, any wavelength of light can have a different amplitude