as someone who does not have a good DAW to mix on, no quality monitors (and i'm not about to port things back and forth to my stereo until its just right...), I find doing the matrix in the field pretty easy, and instantly gratifying.
yes, w/all that stuff i dont have, i'm sure I could produce more "just exactly perfect" mixes, but I cant' think of a single M-S recording done on the fly that didn't please me.
the v2/3 does it w/a jumper setting inside. one for each channel.
my new MiniMP does it on the fly w/a fancy output knob for adjusting width (I think...i'll know more today when it arrives and I take it apart...)