The premise being, you like your friends, they like their friends, and therefore, you should like your friend's friends.
On the ideological level:
It was just simply different way-back-when; when morality in America was more standardized, homogenized. But with the onset of social change, our tolderant attitudes toward those who believe in everything from premarital sex (I'm a fan), to interracial adoption (also a fan), to the death penalty (I vote no), our social networks are getting more confusing. Can't the people with opposing views carry cards, or something?
On the interpersonal level:
No one likes everything about their friends. And these people, being seperate human beings, can tolerate certain things that you can't. I cannot get along with people who are disloyal, dishonest or impolite. But even "polite" I'll bend a little on, and I happen to know that some of my close friends put up with things from their own close friends (putting them in dangerous situations, putting them down, being competitive, etc) that I would not tolerate.
Clearly, "mixing friends" can be a mixed bag.
Case in Point:
OC this weekend was lovely. I didn't go to Seacrets once, and didn't want to. I was staying in a beachfront condo wth K and her roomate, (let's call her Kristy). Kristy's long-time guy friend passed out on our couch Saturday night, then took Kristy, K and I to Bayside Skillet the next morning before spending the rest of the day with us. This concrete salesman and Salisburian was the perfect gentleman.
Alone on adjacent hammocks later that day, he and I began discussing the intricacies of life. Cuz that's just what you do on hammocks. (Or maybe it was the Dirty Bananas that we were using to "hydrate" our burning flesh).
"It's so weird, Roar. I know this guy. He's like a brother to me. He barbacked for me for years. Then, when he got into Maryland, we got him a job barbacking at Bentley's. But come to find out, he's in jail. Last year, some kids were taunting him, so he lit a broom and set the house on fire."
"Yeah, I know. I sang at the Memorial Service for the student who died in that fire."
I think he had expected me to sympathize. But obviously, I couldn't. I wasn't mean about it. But while our perfect gentleman wondered how his friend could have kept the secret so long, I couldn't help but verbalize:
"Maybe the court would have gone easier on him if he had confessed at the time. Someone turned him in, right?"
We went into more detail, but the gentleman eventually left it at this:
"Well, Roar, I had no idea. I'm sure you have a different perspective than I do, given the people you know."
OK, so this is an extreme example. But it is also extremely true. Don't get me wrong, I don't think this guy was a bad guy just because he knows someone who's in jail. But the circularity of this scenario was uncanny. How does life happen like this?
And more to the (general) point: Is it any wonder that the friend-of-a-friend test, these days, is anything but foolproof??