Question for people better networked than me

I created an account on Linked In about a year or so ago mainly because people I used to work with kept inviting me to join their network, and since my job situation is always precarious I figured it couldn't hurt. I haven't done much more than upload my bare-bones resume and add people when they request to be on my network. Generally those folks are people I know from work, but I'm starting to get requests from people I don't know--colleagues of former colleagues and that sort of thing. Is there any reason to say no to these requests? I mean, if it were Facebook I would say no because I don't want my page cluttered with a lot of crap I won't read, and every so often I do post semi-private things on Facebook, but as far as I can tell the whole purpose of Linked In is to expand your network for job-hunting, right? Is there a reason I can't think of at the moment why I should not add anyone who requests it to my network?