I agree that when one is dealing with a wide gamut printer such as the current Epson Pro line of printers, one should use ProPhotoRGB as the working space and print using a profile for the printer/paper combination.
See the post on soft proofing workflow if you haven't already:
http://forum.luminous-landscape.com/index.php?topic=106264.0;topicseenThe point is, few of us know when or where we will output or
master files as Jeff correctly calls them. It might be a Frontier today, an Epson P800 tomorrow. Depending on the raw processor, the gamut potential will be ProPhoto RGB. So expect in rare or special cases, most of us don't know what output device an image might land upon and ProPhoto RGB makes the most sense (to me and others) as the working space for the master image.
However, when looking at the profile of the Fuji Frontier LP7700 printer at my local Costco, the printer gamut exceeds that of sRGB by only minor amounts in the yellows and teal greens as shown here. The profile was downloaded from the Drycreek.com web site, which supplies profiles for various labs.
I can't speak specifically to that profile's gamut or the printer of course. But I've provided gamut maps from devices I have measured and they exceed sRGB gamut enough that sRGB isn't an optimal working space for those devices. Output to the web or mobile devices, fine.
Here's a plot of a LightJet on Fuji MATT paper versus Adobe RGB (1998) and as you can see, the LightJet is larger in gamut in some areas. Why clip those colors?
Here's a Frontier I again profiled (using ProfileMaker Pro like the above profile) again compared to Adobe RGB (1998), again, some clipping albeit as you suggest, not much.
Again, I can't speak to the gamut plots you've produced, only the ones based on profiles I've built and they appear to show that even Adobe RGB (1998)'s gamut isn't large enough to totally encompass those output devices. I'd be happy to send you the profiles if you wish.
Based on what I see above, based on how I process my raw data and with the product I use, based on I might output a master image I've spent hours working on, I'm sticking with encoding into ProPhoto RGB.