A few years ago, when the HDR inkset was launched, I was discussing an issue regarding rendering of certain colours with Eric Chan. To cut a long story short, he had an image that didn't print too well on a 3800, but when I printed it using an HDR inkset machine (think it was a 9900, but honestly don't remember), it was significantly better and the stepping/graininess went away.
That was a particular image and a particular colour, which is the key point. There are some instances in which it will assist a photographer. For proofers trying to hit spot colours, it can be of assistance more often, of course.
It's just that the whole tale is not told by gamut boundary maps nor even just by the number of inks. The Epsons have a massive LUT that they use that will choose different ways of rendering a colour depending on a host of factors. That's perhaps even more important than the absolute gamut volume and boundaries and the internal gamut "depth" derived purely from the inkset colours.
All that said, of course as Mark says there are going to be a lot (most? - depends on the photog and their subject preferences, I reckon) of images where it matters not one bit? Absolutely.