Are you of the opinion that the 2 additional ink colors of the L1800 are superfluous, for marketing or profit purposes?
There is no simple answer - it is a kind of everything.
You need to view the light inks - or photo inks as Canon calls them - together with the droplet size of a particular printer. You don't gain any gamut from those, they are diluted inks. Larger format printers - Epson or Canon - use a droplet size of 4pl, and ink drops are less visible from dithereing when the ink is thinner, lighter than regular ink so you get a smoother , less grainy print - at a very close up view , yes there is some benefit in this respect but it is normally difficult or impossible to test that on the same printer - printing with or w/o light inks. I could do it longer time ago on my Pro 7600 with some RIP software, it allowed me to print with or w/o the light colors by a simple software flag in the RIP software. And there is another parameter you need to take into account - the viewing distance - you typically view posters from a greater distance than 4x6" prints , so yes - you can see a difference at a close up view - which is not typical with poster sized prints.
When you look to desktop printers the situation is slightly different, Canon was/is using since a long time - more than 15 years - a droplet size of 1 - 2 pl and a 5 colors inkset in most of their printers -2 black inks, and you can get great quality prints from these printers, no reviewer complained about quality issues from the lack of photo inks.
Epson configures the printers lightly different - the multipurpose printers - expression home, Workforce etc come with a printhead with 3-3.5 pl droplet size and 4 inks in most cases. You can get very good prints as well with some non-standard driver settings, and you may not see any graininess if you are not aware of it , or you see it if you know about it and look closely. Epson is offering separately some printers models - Expression Photo and such - with an extended 6 color inkset and a droplet seize of 1.5 pl - XP55 - or with 5 inks and 1.5pl like the ET7750. Both light inks and a small droplet size together is becoming an overkill. Again - you normally cannot run the same printer either with 4 or 5 or 6 inks - I did a test with an 6 ink L805 - using the regular inks in the light ink channels and tried to run a profile with it, it is possible, but there are some color quirks in a transition range between the regular and the light inks since I cannot influence the driver internals. But this allowed me nevertheless to print light colors - the upper range of the gamut - you need to look very careful with a loupe to see any differences - whether a light color was dithered with light inks or regular inks - the drop density is slightly different, but that's all below a regular threshold level of visibility. Canon can do very well w/o light colors, and the ET7750 as well.
And you have a class of printers - like the Epson P400, P600, 700 or the Canon Pro10s or Pro100 which are above the A4 desktop printers but below the large format models , Epson is using light inks - P600/700 or not - P400 all with 1.5pl droplet size - Canon is using photo inks in these printers with 4pl droplet size.
That's technical details but you really would need to compare actual printouts to get a final visual impression.
I did that some time ago with 2 forum members, we exchanged Canon and Epson prints, on the same paper , I did remote profiles for the other user so that we wouldn't get into profile issues, lightness etc, and we were surprised how good and close prints with an Epson and Canon printer could get, and this with refill inks at our choice.
