There are a lot of good reasons why you should only send LinkedIn connection requests to those you know, but here’s the best one of all: attempting to connect with those you don’t know is a violation of LinkedIn’s User Agreement and doing so can result in your account being restricted. Here’s the official slap on the wrist you’ll get from LinkedIn, which is triggered when your connection requests get an “excessive number of ‘I don’t know this person’ responses”:
The first time your account is restricted, it’s pretty easy to get it reinstated. However, if you continue to violate the terms of the User Agreement, “your account may remain permanently restricted or be closed by LinkedIn,” as the warning above states.
So, it’s not just an opinion that connecting only with those you know is the best approach–it’s a fact.