LinkedIn is a notorious spammer. Any useful networking services it may offer pale in comparison to the fact that it is perhaps the worst spam site out there. Or, as a certain blog comment says, no, they're really not such a useful site.
Yes, LinkedIn offers an opt-out link. However, you have to register in order to opt out. I don't want these purported "networking sites" to have my details. Facebook used to be one of the worst spammers out there, although they've received enough scrutiny lately that maybe they're cleaning up their act. LinkedIn certainly is now, and I resent that people give them my details and I certainly won't volunteer any of my info.
However, as bex77 points out in a comment, LinkedIn spams the mailing lists of people who sign up. I had no idea, and this bit of information has just greatly multiplied LinkedIn's ick factor, which was already pretty high. I may try to contact people whose accounts keep sending me invitations and reminders, in case they are not aware.
The fact that I have to go to such lengths to avoid getting repeatedly spammed is ludicrous. LinkedIn's attitude towards all this is reprehensible.