I don't use these services, so I'm probably wrong, but don't lists and circles solve this problem? While "friending" may be binary, you can adjust the connection to the other person/profile by moving them from e.g. "close friends" to "acquaintances" and therefore give them less or more access to your profile.
I think the issue Kevin has isn't their access to his profile, but his exposure to their content. When you're in high school you care about what your high school friends have to say, but 5 years later you don't care about what most of them have to say any more, but manually removing them is inconvenient and not caring what they say is not directly tied to being "friends" or not.
I think the solution he's looking for is something that analyses your interactions with a specific friend and the common interests and hides / reduces exposure to their content over time. Although I was under the impression Facebook already did something similar to this.
