I changed my privacy settings set on LinkedIn so that no one could see my connections after I started noticing that many recruiters sending invitations were already connected to other people I know or have as a contact otherwise. I strongly suspect some of these recruiters just want to add you for the sake of having access to your contacts lists in order to have more people to connect with.
I don't usually accept requests from recruiters, either. In fact I had this for a while as my first sentence in my profile page and that didn't stop them from sending requests. That signals me they don't bother checking your profile first, either.
Apart of maintaining my vanity URL, I don't even know why do I still have a LinkedIn account; I just don't find it very useful.