While I largely agree, the direct messaging function is also very useful - sometimes I can't find the email/number for the person I want to speak to because they've moved on since I last had their details and for whatever reason I didn't get the new details - in that case LinkedIn messaging is often a good backup and often a fair bit quicker than asking a series of mutual acquaintances for details.
Definitely not. No one I know uses LinkedIn except when they are looking for a new job.
The ones I've found who are using it a lot are either: paid to, or desperate for work and try to fluff up and BS their way into a job.