1) You don't need a chord for every note- contrast is good.
2) 3rds and 7ths define the basic chord sound- good to have on the lower two strings.
3) Voicings in 4ths (and 5ths) sound great on mando (and everything else). Grapelli often harmonized his lines in 5th-easy to do, too!
4) It is useful to break away from the idea of "chord blocks" and to see the other notes of the scale of the moment as source material to harmonize the melody (or to create counterlines).
5) Doing this kind of arranging/improvising gives you a broader view of your instrument.
6) Improvisation is spontaneous composition. You can also improvise in slow motion as you create an arrangement-but it helps to take notes (or record) as you go, so you can remember what the hell you played!
The more chords/voicings you know, the more melodic material you have to work with. Plus, it's great fun! :twisted:
I've never heard Django play a note without commitment.