Look at Figure. 3b (about half way down) ProPhoto RGB "actually exceeds it (visible spectrum) in the deep greens and deep blues. What this means is that colours can be pushed into areas which can neither be seen nor reproduced, producing very nasty looking results within the visible spectrum. User beware."
Well I don't know if user beware is compulsory, we don't want to scare people because its necessary to have such large working spaces.
Forgive my standard copy and paste, but it puts these kinds of color spaces into perspective.
I call it, Gamut mismatch (fitting round pegs in square holes)
It IS true that the wider the granularity in a color space, the harder it is to handle subtle colors. This is why wide gamut displays that can't revert to sRGB (current LCD technology doesn't allow this.) are not ideal for all work (ideally you need two units).
There are way, way more colors that can be defined in something like ProPhoto RGB than you could possibly output, true. But we have to live with a disconnect between the simple shapes of RGB working space and the vastly more complex shapes of output color spaces to the point we're trying to fit round pegs in square holes. To do this, you need a much larger square hole. Simple matrix profiles of RGB working spaces when plotted 3 dimensionally illustrate that they reach their maximum saturation at high luminance levels. The opposite is seen with print (output) color spaces. Printers produce color by adding ink or some colorant, working space profiles are based on building more saturation by adding more light due to the differences in subtractive and additive color models. To counter this, you need a really big RGB working space like ProPhoto RGB again due to the simple size and to fit the round peg in the bigger square hole. Their shapes are simple and predictable. Then there is the issue of very dark colors of intense saturation which do occur in nature and we can capture with many devices. Many of these colors fall outside Adobe RGB (1998) and when you encode into such a space, you clip the colors to the degree that smooth gradations become solid blobs in print, again due to the dissimilar shapes and differences in how the two spaces relate to luminance.
So, since we're dealing with simple shapes here (synthetic RGB working spaces), you have to move the primaries father apart, in the case of ProPhoto RGB, that means two are outside human gamut. So yes, you can define colors you can't see, let alone work with colors that fall outside your display gamut. Its just the compromise we have to deal with working with large gamut encoding color spaces. But I don't know that we need to be too scared of this (user beware now is probably appropriate)