I'll try to explain what I view as the advantages of liking people.

It's instrumentally useful to give people the perception that you like them: they are more likely to like you, and to want to cooperate with you. Probably the best way to give people the perception that you like them is to actually like them.

There are qualities in people that it is unwise to overlook, because the cognitive dissonance is so strong. "He's fun, except for the light stealing," is not going to lead to healthy thinking. This is inevitably corrupting.

Yes, I agree. As someone with high Agreeableness and Openness, I've been burned in the past for being too trusting of people. This tendency is why people with high Agreeableness need to learn certain skills (alluded to by SarahC, including a healthy amount of suspicion. Similarly, Disagreeable people may need to learn to be more trusting and open towards people. Otherwise, even though they might avoid getting burned, they might shortchange themselves on positive interactions and connections with people.

An emotionally Agreeable person applying cognitive cynicism, and an emotionally Disagreeable person applying cognitive openness, could have the same estimates of people's trustworthiness; they are just coming from different routes.

Anyway, it's a very interesting post, much as I think it's a bad idea. I note that I was and remain a big fan of niceness in most circumstances and a big fan of the niceness post linked at the top of this one. I think this is dangerous step past that.

Well, the best way to be nice is probably to genuinely like people. I agree with you that adopting such an attitude has risks; I just think that if you can mitigate those risks, an attitude of Agreeableness combined with some cognitive caution towards people and their motives, is a powerful combination in our society.