I've gotten interviews, and my most recent job partly through linkedin. It's actually now a red flag, at least in heavily-white-collar professions, not to have a reasonable professional network set up. It might not cost you a job to not have one, but it will probably be asked about in interviews. Things like security clearance investigations now focus a hefty amount of scrutiny on your on-line activities, too.

I guess it's a tool for lazy recruiters who feel better about passing you through to the next step if they can see your glowing endorsements and some professional engagement with your peers.