If they've looked at my CV/Profile it's validation that my skillet is in demand, and that's always moderately good to know.
If I'm looking, or even considering looking, then it's awesome - I've got a lead into a job before it's advertised.
If I'm not actively looking (or considering looking) then I get some information about the market that may well be useful for when I -am- looking.
If I'm not actively looking, I tell recruiters what sort of position would turn my head. Now, my current role is pretty damned awesome, but if they can offer something better, then I want to hear about it.
It may well be different in other parts of the world (I'm in Australia, and most of my experience has been here or in New Zealand), but I generally find that recruiters offer a service that (as a candidate) I value.
Ironically, as the person doing the hiring, I've generally found them to be a waste of time and effort.