I wonder this too. As a relatively young professional, I regularly have folks add me who are my age or younger (mid 20's), and have a portfolio of experience that reads as if they are a Fortune 500 CEO. Combine that with having 5000+ "connections", a slew of unverified and uncorroborated skills, and LinkedIn continues to feel frivolous.
Yet most jobs listing these days require a link to your profile, so it's clear there's some value in having a "complete" profile.
There was an article on HN a few weeks ago about a guy making a game out of LinkedIn. In that he made a fake account, filled it with buzzword skills, degrees, and colleges. Started adding every notable person he could. Then it became a domino affect. Once he had some influential people as connections, other people started adding him, thinking he was then a big shot. Then once he got a ton of influential connections, recruiters from major companies started reaching out to him with job offers and interview requests.
Ultimately LinkedIn does have a good business use, but it can also be gamed pretty hard.
EDIT: Found the link. It didn't originate on HN, but I saw it from a post about it on HN.
Yet most jobs listing these days require a link to your profile, so it's clear there's some value in having a "complete" profile.