I know that some people do benefit from the business contacts that they make through LinkedIn, but I feel like most maintain account 'just in case'. A couple of years ago after realizing that LinkedIn really didn't matter in my field, I deleted my account. I doubt they browsed your contacts without permission. But I wouldn't put a large sum of money on it. The again, maybe they scrape your public info, or buy it.
Also, your "just in case" scenario is right on the money. I update my resume there about every 6 months. Otherwise, I may forget to. If you spend 5 or more years at a company and then all of a sudden are back on the market for a job, it may be difficult to recount all of your accomplishments over those 5 years. If you update your LinkedIn account regularly, you will be prepared for such things.
khaaan, About 4 years ago I hated my job. I sold food to restaurants for a company that pressured me daily to develop tight bonds with customers and then charge as much money as I could for products. I was good at it, but I HATED it. Loathed it. One of my friends told me to create a LinkedIn account and post my resume there. I did. About 3 months later I had all but forgotten about the LinkedIn account, when all of a sudden a recruiter contacted me. He had seen my resume on LinkedIn. I actually think LinkedIn is a great idea and can work well for people in the professional world to network. It is NOT a social site, it's a professional site. That said, I have had a number of people from High School try to "connect" on linked in and their job title is "domestic engineer" or "bible study teacher". While both may be interesting pursuits and perhaps even important, depending on your stance, they're not appropriate for LinkedIn. What I've seen of late is the Facebook-i-fication of LinkedIn. I love my current job and I would have never found it without LinkedIn, an ambitious sales recruiter and about 4 interviews.... but it all started with LinkedIn.