LinkedIn, like Google and Facebook, benefits from the more it "knows" about you. As I and others have frequently mentioned, you are not the user of such services… You are the product that the service "sells." In LinkedIn's case, the more information they have about their users makes the paid services they sell more desirable to those who choose to use them (especially recruiters). How they weigh the information in profiles to get to 100% is (if they are smart, and I know they are) based on getting the most valuable information entered by their users.
The LinkedIn UI design that urges users to get to 100% complete in their profile, by the way, is a very good example of "gamification." We all want to "win" and when we see something is incomplete we are motivated to do what it takes to get to 100%… and so we fill out the missing information.