If I understood well it seems that when your justice-meter is filled up you consume it as soon as you use any violet weapons.
If you equip DeathSpank with two violet weapons you have then no choice to keep Justice for the right moment. If you instead equip him with 1 violet and 1 non-violet weapon you cannot build up combo multiplier using your weapons alternatively.
Is really designed in that way or am I miss something? Is it possible having more control on your Justice Ready! and decide when really use it?