The additional inks are NOT warranted whatsoever by Epson. They will fade much quicker than the standard CMYK inks will, resulting in much shorter lifespans for graphics.
I remember reading this 7+ years ago when we got our S80600 and i was freaking out that i just bought a machine that was not suitable for our needs. So I printed a few 12" squares of solid red, orange & CMYK, I installed 1 set on the roof of my building, facing up at the sky, 1 set on my back door which faces south, and 1 set i kept in my desk drawer as a control, after 2 years, the one on the roof was looking crispy, but the colours were pretty close to the control, and the set on my door were virtually unfaded, after 4 years the ones on the roof had turned to dust lol, and there was a bit of fading on the set on the door, but it still looked fine until you held it up against the originals.
I don't worry about it any more.