It's networking: I love almost everything about book publishing, but I have to acknowledge to my customers that the permissions game--that is, covering yourself by asking permission to use any material you want to quote--is a pain in the p'toot. It's mostly time-consuming paperwork, combined with the dread that you might not get permission to use an important quote--or, worse, be asked to pay for it. The person on the other end--the one who will give you permission--probably isn't having any fun either. (I speak from experience.) The only reason you go through this process is because it's the ethical thing to do (or, if the ethics aren't driving you, because it could keep you out of some big-time legal trouble).
Unfortunately, this sort of honesty isn't particularly motivating to authors. Quite the opposite. I was working through this dilemma with a would-be author when she pointed out that many of the people she was quoting were potential allies that she wanted to meet. In other words, they were "key contacts." It dawned on me that all of us could be looking at the permissions-gathering effort more as an opportunity for networking than as a strategy for staying out of legal trouble.
Think about it. When you quote somebody and give them credit, you're plugging their book and their work. Usually, they are going to be glad you're doing so. Moreover, chances are better than average that they will have a natural interest in what you're writing about. Asking permission to use their material is a great excuse for making contact. Of course, they may turn you over to the publisher to complete the paperwork, but by then you've accomplished two things: 1) talking to an important contact and 2) finding out where to go for written permission. The Publishing Pro.