Earlier quoted context omitted.
They used to give guidance about what 'know' meant and I still adhere to the 'I will only link with you if I have properly worked with you' line. The fact that few other people do have devalued its use.
At some point, LinkedIn changed its mind and decided that you should connect to anyone you're barely acquainted with and/or have exchanged an email with. They're actively encouraging forming these "devalued" links. They even use shady practices like borderline forgery of invitations.
For this reason, I'm somewhat lenient in adding LinkedIn connections, as long as I at least have some idea who the people are. Unfortunately, many connection requests are from people I've never met before.