I am also curious about this. I have used the sp404sx with my modular before.
One way in which it was useful was as a 2 channel drone source. I made samples with sound panned all the way left or all the way right, essentially giving me two voices on two outputs. In order to have it make sense I placed all the samples panned left on the 6 leftmost pads (1, 2, 5, 6, 10, 11) and the right ones on the right.
I routed the two channels through filters and vca’s enabling me to create rhythmical elements from these drones.
Here is a short clip of this in action (although not the best audio quality): http://www.thepictaram.club/share/BVQOFysAWed