I have to say that I think you have become a little nervous about pinning your opinion to the fence. I understand why this is the case, I have watched you get battered for posting opinions on image quality of various products that other people dont agree with but thats the nature of it, stand up and tell us what you think. As a working professional I understand that there are many other things than IQ that come into play when deciding on equipment but IQ is very important, dont shy away from your opinions on it.
The challenge here is those very minor differences are subjective, and dependent on many factors (which paper you like for example). I believe he stated that IQ most likely would not be a useful measure by which to make a purchase decision. After using both the ipf6100 and 11880 for several months myself, I believe this is a completely accurate and fair assessment. I know people don't like to hear this, and it sounds like a cop-out. Everyone just wants to hear which is better and why so they can make an easy decision.
From where I sit, with very little bias or loyalty to either machine and using both frequently without much regard to which one I use (just depends on which location I'm working at) I would agree completely with Michaels assessment. Both printers produce incredible quality, and even side by side direct comparisons are virtually indistinguishable from each other. Do I believe one is better? Yes, I believe for a perfectionist the Epson printer is very slightly better ... Michael used the word quibble and that's pretty accurate as far as the difference.
There are many other factors to consider, but in the case of an 11880 to x100 series printer, I believe there is only one major one, cost. Ink clogs, ink swaps, and even speed do not factor into the decision between these two printers anymore. The 11880 at 2880dpi and uni-directional printing is pretty much flawless, with no visible dithering an any tonal range, perhaps slightly sharper in some limited circumstances, and with perhaps a slight edge in detail in low contrast shadow and midtone regions.
Is that quality difference worth $11,000? That's again personal. If you are a perfectionist, and need the mental reassurance that there is no way to get a higher quality print ... despite the fact you probably can't visually see it, then perhaps. Maybe it helps you in selling your services, and you charge enough to justify the machine. In my case I bought it because I'm one that just always wants to own the latest and greatest, cost be damned.
Of course, this is just my humble 0.02 opinion. Probably not much help.