It will also allow them to argue that subject to compliance with the licence, the information could only have been accessed from CL.
The reality of course is that for a certain number people (perhaps limited) CL will be the only site where the submit content and will therefore not be concerned about granting an exclusive licence.
However, I'm sure a greater percentage of people will not necessarily grasp what Craigslist are asking for here and will not be put off posting elsewhere. Of course, CL will not go around enforcing the terms of the exclusive licence against the user.
The grant of exclusivity will simply allow them to argue that subject to compliance with the licence, the information could only have been accessed from CL, or at least is more likely to have been obtained from CL.
If Craigslist are serious about ensuring they can rely on the exclusive licence it should be made clear that the grant means the user can't post the content anywhere else however this may mean that people would be more put off using the site.
My view is that it's a shame that CL are now attempting to assert an even tighter stranglehold over information originating from users. In the long run it may assist Padmapper in pushing people away from using CL in the first instance.