Is having blind faith in the good of people a desirable quality?

I believe it is true to a certain extent that people believe the motives of the others to be what they themselves would have done. For instance, if you buy someone dinner with a hidden agenda to bribe the person into doing something favorable for you, most likely when someone else buys you dinner, you'll be suspicious of his/her motive.

So when a person believes unconditionally that every person is good by nature and what he seems to be doing is bad might actually be something good beneath the surface, is the person in question the kind, good and honest? Maybe.

But is that a good thing? Shall we imagine a scenario as follows:

A: C refused to let his parents stay with him because he is selfish and unfilial
B: Maybe C had no choice. Maybe C is broke. Letting his parents stay with him will only cause his parents to suffer with him. At least now the parents have the welfare funds to ensure their daily needs.
A: C just bought a new car yesterday. Do you mean C is too broke to take care of his parents but not too broke to buy a car?
B: Well... maybe C's parent's weren't nice people. Maybe they ill-treated him when he was young. Or they hadn't fulfilled their responsibilites as parents in the first place?
A: Anyone can tell you that C's parents love C to bits. Everything C wanted, the parents gave him!
B: C might still have some unknown but valid reason for doing so...

Frustrating isn't it? Talking to someone like B. But aren't those very good virtues, to always look on the bright side, to not judge until it's proven cold and hard that it's a fact? But sometimes we can never find out the truth with conviction. We need to weigh the possibilities to decide for ourselves which is more likely the truth.

So we're back to the question: is the extreme of blind faith the way to go or a neutral rational way of seeing things more practical, more sensible? I do not have the answer to that. Just like I do not have the answers to many things. But I hope it'll make you think twice about the kind of person you want to be the next time you judge somebody.