Foucault's counsel doesn't make sense, in that no one has ever said that "one has to be *sad* in order to be militant." I'd think that one has to be *angry* against injustice, exploitation, or oppression (of at least some kind, even if s/he has yet to see the ensemble of social relations that creates & recreates it) in order to get involved in political actions. Overwhelming sadness depresses you, making you inclined to turn inward & withdraw from the world, sometimes to the point of paralysis. In contrast, anger -- if you are not consumed by it -- motivates you to do something about the object of your anger (e.g., racism). The difference is rooted in biology: "when you get angry, your heart rate and blood pressure go up, as do the levels of your energy hormones, adrenaline, and noradrenaline.... Anger is a natural, adaptive response to threats; it inspires powerful, often aggressive, feelings and behaviors, which allow us to fight and to defend ourselves when we are attacked" (at <http://www.apa.org/pubinfo/anger.html>). The point is to control anger & use it productively.
Yoshie