Where lies the value in convincing people who afterwards won't be useful for "our cause" anyway (we're talking fAGI I assume)?

It's called "getting vote and then passing the bills". We get people to vote for us for what they thought they heard us say. We'll pass the bills that are about what we actually said. Doing away with metaphors: we get them to actually listen to what we have to say by giving a great first impression, then once we have them captivated we start showing them the fine print, most notably the bits about how much they suck and they need to change and if they listen to us everything in their lives will be better and they'll be spiritually and emotionally more fulfilling and awesome. Parties do this. Religions do this. Universities do this. Parents do this. Lovers do this. Why should we be any different? Once we get to the point where we can persuade them that talking about rationality in clown suits is a perfectly reasonable idea, the rest is pretty much done.