I like the D on string 9, with a 10 string E9 tuning as it is.
Beside the dominant 7 for blues, it enables a lot of jazz chords as well.
On 12 string, I do really like the universal tuning without the D.
I quickly got used to using the lever to lower 8 to D when I needed it as Larry mentions.
Im not much of a fast picker on the low strings yet, but work on it when I can.
Franklin, thanks for the links! Tremendous players I had not seen before.
I changed my 12 from Uni E9/B6 to Ext. E9 recently, to compare the two tunings.
I do really like the 3 and 6 G lower a half on pedal 0 as Jerry mentioned.
Id keep that pedal in a Uni setup perhaps as well.
Oddly I am having a little harder time with the string 8 D on 12 strings than I do on 10 strings.
Maybe the string spacing has something to do with it?
Adding those 2 low strings also introduces a few grips Im just not used to yet.
The Uni grips felt natural right off the bat, and helped me ease into study of the C6th tuning.
For some reason the lower tuning of the universal was just easier to grasp for me.
I may change it back to U12 E9/B6 again in the future, but I'm having fun
trying to play the Ext 12 with that now pesky D string haha.
Clete