I would say that now, about half my LinkedIn invitations are coming from marketing trolls who obviously want to sell me something. I deleted three invitations this morning, from an insurance rep, a CPA, and a car repair shop. I don't know of any of these people.
LinkedIn seemed like a reasonably modest "good idea" when it started, but I can't say I have ever used it for its supposed intended use--networking.
Here is the problem. You can accept an invitation from almost anyone, but you will quickly be inundated with LinkedIn messages asking for a minute of your time, and there are only so many minutes you can give away to LinkedIn contacts you don't know.
If you are very careful about who you accept as a LinkedIn contact, then you are using LinkedIn to stay in touch with people you pretty much already know...so you already have their contact information and don't need LinkedIn at all.
It's a mess.