To me, Facebook is sort of the online equivalent of a big social function, like an open house. Some people invite the whole neighborhood, some people just invite close friends and family. Once they're all in your house/feed, you can listen to all the conversations your friends are having, and have conversations with them. I unsubscribe when I'm tired of listening to a person's conversations, but I don't want to go through the trouble of kicking them out of my "house" by unfriending them, just for being annoying.
Sometimes I check back in, to see if they've gotten better about the drama-mongering, sales pitches, quotation pictures etc. But I don't usually defriend unless their behavior is really egregious.