In Onyx 12 when you build a profile using only CMYK, it still uses the Lc and Lm.
That is not correct. You do have to have the printer configuration set up for CMYK as well as 6,7,8, etc colors.
I have been profiling in Onyx since version 2.
I know exactly how it works.
I just did 2 customers who had bad heads, one was light cyan and one was light magenta and they didn't want to spend the money replacing heads on the old EFI and the old Agfa UV printers and the banding went away because the light colors are not being used.
The cost reduction happens when you are printing mid-tone and light colors.
It's not good for smaller items printed because it will be grainy since for lighter colors it will just make fewer drops of the CMYK heads.
This method is used primarily for large truck wraps, full walls, etc.