[Taylorfanguy]

I use them both often. They both impart subtle sonic derivations and musical nuances when used appropriately. It's the same reason that people use Em and Em7. Don't get me started on broken and partial chords. There are literally almost infinite colors to paint with on that spectrum.

Here's a homework challenge : learn to play variations of the A chord in nine different ways on the fretboard. Then do the same thing with the D chord.