Yes out of grp char as DPS does work, better with a caster than a melee, and best if your dps toon has "heals=off".
It is VERY beneficial if you put you MA in Xtarget.
If outside DPS is pet class, you must have toon do something (cast spell, throw something, attack somehow) to put mob on Xtarget in order to get pet to assist.
Melee classes outside of group tend to be a bit slow in the assisting, not sure what is exact reason, but they just seem to delay 10-15% longer than normal.