George, knowing your love of diagrams, I have created a couple to try to explain visually what I cannot seem to get through to you in words. I have created a fictitious slice of part of a 2-D representation of a couple of colour spaces. The boundaries happen to be straight lines and the colours are what they are. What I am trying to demonstrate is why bit depth matters (more) in a wide gamut colour space when it comes to real life editing issues. Ted, this has nothing to do with all the various colour spaces you brought up in your questions, because it really doesn't matter, and no, it is not a perfect analogy to the real world issue I am trying to describe graphically.
In the first diagram I have created two "colour spaces". The first one is a "narrow gamut colour space" and the area shows a limited range of colours and colour depth. Each of the squares in a 6 x 6 (which represents the bit depth) is more or less a contiguous colour and when I look at it visually each square is more or less the same colour.
When I apply the same 6 x 6 grid to a "wide gamut colour space" to the larger square the squares are no longer a relatively pure colour. This is especially noticeable when we see where the green and red colours start to interplay with each other. This is the real world issue I have been trying to explain; a small bit depth is not granular enough to represent the individual colours in a wide gamut colour space and we run into technical issues when manipulating the data that results in artifacts.
The second diagram shows the same wide gamut "colour space" that I used in the first diagram. The square on the left is identical to the one on the right hand side of the first diagram. The square on the right is identical to the one on the left except that rather than a 6 x 6 grid, I have overlaid a 24 x 24 grid. The right hand grid represents what happens when we increase bit depth and the amount of data we have increases. The colour in each individual grid element is much more consistent than what we see in the 6 x 6 grid. Increasing the bit depth gives us more granularity, which in turn lets us define the colours more precisely. This mirrors what happens in the real world when we use a higher bit depth; we get little to no artifacts when we use a higher bit depth when using a wide gamut colour space.
If bit depth does not matter, think about cutting the 8-bits in half and try to describe all the colours using a 4-bit definition. With 4 bits, we get 2 x 2 x 2 x 2 = 16 bits per channel; or with 3 channels; 16 x 3 = 48 colours. So bit depth does matter...