The gamut is no larger (light cyan is still cyan, light magenta is still magenta, both add precisely zero to the gamut) but you can achieve slightly smoother gradations in certain tones like light skin tones very light skies, etc. The difference for 98% of people will be unnoticeable. If your primary market is vehicle graphics, banners, displays, signage, etc then do not bother with LcLm. If you specialize in prints for professional photographers and artists then you might consider 6 color.
To be honest - we have always run dual CMYK and over the years I have done plenty of work for both artists and pro photographers and have never once had a complaint about quality.
Now if you can find the ink and a RIP that will support it you could run something like CMYKOG (orange and green) - that will greatly expand the gamut. Mimaki does have some options in their SS21 inks, they have light cyan, light magenta, white, metallic silver, and newly introduced light black and orange. You would need their latest version of Rasterlink to run many of these configurations though.