There is no practical reason. I just want to see what colors is my camera (after post processing) able to produce. I know there are at least some colors outside of ProPhoto, because I can see some clipping when looking at some color plots.
I understand. Do realize though, that the color plots are based on approximated RGB coordinates that change when e.g. a lower saturation is used during Raw conversion. You are almost certainly not looking at spectrophotometrically correct color coordinate positions. Also, the larger the gamut is, the larger the quantization steps between subsequent integer number coordinates (that's why I mentioned that floating point numbers would be required). Also, most actual images do not fully use the full gamut space, and in fact Prophoto RGB also has the room to encode non-existing colors that will therefore never be used.
The attached image shows a lot of colors from a shot of some trees (the picture is mostly green) into xyY coordinates. This is a ProPhotoRGB image (from LR) and the red triangle shows boundaries of the embedded profile. There is some clipping of the greens, less extensive but similar as what I see with plots made from images converted from ProPhoto to colorspaces like AdobeRGB, so I guess there is more to see if I use appropriate converter (and more colorful captures).
Yes, good example, although the xyY plot only shows the coordinates at a certain Luminance level, and the space is not perceptually uniform (the issue may more, or less, significant when shown in e.g the CIE 1976 (L*, u*, v*)
color space), especially in 3D.
What you probably want, and you'd not be the only one, is a better Saturation clipping indicator (not only that it's clipping, but also by how much), and tools to convert between (or specifically towards output) colorspaces.
As other threads here on LuLa and elsewhere have demonstrated, the Raw data may be unclipped before White-balancing, but combined with a given exposure level that data may become Out-Of-Gamut (OOG) after white-balancing. So the actual Raw conversion may already address some OOG issues later in the workflow, without the need for a wider gamut colorspace.
The drawbacks of an 'Mega' large colorspace (e.g. posterization risk with large manipulations and inaccurate colors due to sparse linear quantization with integer coordinates), may outweigh the benefits for a few colors that could be addressed/tamed in a different way.