What is the reason that third party recruiters are so persistent about obtaining a copy of your resume? Is it worth working with a recruiter that contacts you about a retained position?
Its part of the file they keep on you - if they have a position in the future, they may call you up if the think you're potential candidate match or if they want to check-in on your to see where you're at. I get those calls all the time.
I don't understand what you mean be retained position. Do you mean contractor? Irrespective if you you're interested with the job, then no harm done by talking to them. So far my last 2 jobs are based on being head-hunted by recruiters on a mandated search (i.e. executive search firms with exclusivity to fill the role) - and in my career mainly I have to say I've been "found" vs. hunting. All good in my books if they play a role with helping you get introduced to a company in a good light - the best you can hope for with a recruiter relationship.