Digital BW, The Print

I have been experimenting with the MIS UT7 inkset on a SP2200.

I am familiar with getting QTR to work with UT2 (Ultratone for QTR and 
Bowhaus) inks and was interested to see that the UT7 inkset only 
provides 3 Black inks while having 3 toners and a "custom light black"

I know that these inks were originally formulated for Roark workflows 
which might explain the ink choice.

My questions:

Are there any visible dis-advantages to having only 3 black densities? 
It seems like this could be less desirable in theory than the 4 black 
densities of the UT2 inkset.

Why would one use the "custom light black"?

Why would one choose the UT7 inkset over the UT2 inkset for QTR?

James Haney