Haha - I agree with all the points you make but still find it useful. It depends on your industry and your aims, I guess. It supports my work rather than producing a separate income stream by itself.
For me, it's a good interactive address book for colleagues and contacts and a good resource when someone needs an intro or advice on an organisation someone else has worked for.
I've used it occasionally for my own background research to figure out how certain organisations and companies are arranged, who works for whom, who has direct access to the top, and where the buck really stops for particular workstreams. (If you do too many searches like this, LinkedIn does assume you're a recruitment consultant and ask you to start paying to continue...)
I also like being able to message some old colleagues without giving out my personal email address or Facebook profile (which is strictly family, real friends, kids/cats only). These people for whom I have enough liking, interest and respect not to want to lose touch.
When someone publishes a book, starts up a company or organises an event, I like being able to recommend it to others in my network where I know they will appreciate the work and find it helpful.
Everyone gets a few dodgy randoms. I probably get more than most as I have a very common name and generic face. If I don't know them or their company / work, I just tend to ignore.