Fi users don't "brainwash" people in the way that Fe users do. an Fi user trying to win people to his side of the crusade is more likely to use a more forceful, direct approach with the aid of Te. Fi/Te is a much more aggressive, fiery combination than either of the two functions alone. it is less concerned with persuading people and thinks more along the lines of "either you're with me or you're enemy! repent sinners lest I strike thee down!". ie, they don't give a shit about what you feel, they only care about what you believe and if it is in line with what they themselves do. Fi is too uncompromising and Te too direct to resort to the kind of diplomacy and social give and take necessary for the kind of manipulation mentioned in the OP.
[MENTION=6164]Riva[/MENTION]
Fi users can be manipulative, but they aren't using Fi to do so. I've manipulated people before with Ne/Te, but it's in a different way entirely. basically, I just throw up a really professional, competent looking image, back my zany ideas with some empirical evidence and deliver the message in a confident, authoritative voice.
PS: to be honest, though most of the times I've manipulated people like this have been on accident, as I quite enjoy this demeanor and I have this tendency to be able to sound REALLY competent when I actually have no idea what I'm talking about...only to realize this later and fall flat on my face. I guess I'm still a wannabe TJ 3w4