Good question, Kent and Gursharn. A variety of reasons - spam is one, but more often I find the user complains of not "getting anything" out of the site. Often, they are confused as to how to best use LinkedIn. They expect it to be like Facebook, and it is the opposite, as you know! They post a profile and then wait for "something" to happen and complain when it doesn't. They join no groups, then wonder why their networks aren't growing. They set their profiles to accept no introductions or InMail, fearing connecting to "everyone", and then wonder why no one is connecting with them. They completely forget their professional manners, then wonder why others stay away.
LinkedIn is a simple site to use, but the user must allow for ramp-up time to learn the ins and outs, as with any platform. H/she must actually use the site's features daily - even if just for 15 minutes - to achieve minimal success. Tons of information exists on how to use LinkedIn correctly! Still, LinkedIn is not a good fit for every person.
If you use TweetDeck, create a column to watch the LinkedIn hashtag. It is amazing to see the speed of posts (it appears new users actually DO join every second!) but you'll also see people writing things like "I want to leave LinkedIn but they won't let me close my account". I provided the steps to help these people make a smooth, non-stressful exit.