It's a pretty useful tool if you are in a role where you network for a living - sales, marketing, etc. Or if you are actively are looking for a job and aren't in highly specialized niche with a talent shortage.
How could it be useful in a job search scenario? Let's say you want to work at X company and you have a friend, or a friend of a friend who works there. You ask them to refer you into their company, and since companies typically will favor internal referrals, you are more likely to get hired.
A large percentage of jobs are filled without ever even being posted online, so for most people networking is their best bet at landing a new job.
Unless its entirely unwieldy and you get tons and tons of emails from friends of friends who are hounding you all the time, then its pretty much common courtesy to help someone out who asks you for help on LinkedIn or otherwise.
Being contacted by recruiters all the time is another story, but thats kind of the trade off you make when you sign up for a free professional networking tool.
Don't want to deal with recruiters contacting you but still want to keep your LinkedIn account? Let people know you don't respond to messages on your profile and unsubscribe from emails or create an email filter. Or even just filter out emails from the word "recruiter" & LinkedIn and that will probably catch most of it.