I find all this talk about INFPs and their lack of assertiveness to be very odd. Maybe there's some confusion between leadership and assertiveness endemic in these definitions?
Regarding leadership: I don't mind being a leader, but I'm usually more open to someone else being a leader (usually an Extroverted type) because I can accept that not only will be s/he be good in that role, but s/he will enjoy it more than I will. I have no pride/ego issues in that regard. However, I have no problem being assertive when assertions need to be made. In fact, I have frequently been accused of being bolshy because I'm so obstinate about things when I feel forced to go so far as to speak out against something.