Hi Dania
I guess it depends what kind of exclusivity they mean? If you are hired on a job, for example, the images may just be for sole use by the company who are paying you (although they will probably let you have a copy for your portfolio).
However - if its you hiring the photographer, or working in collaboration (so you both get photos out of it at the end but no money is exchanged) I wouldn't let them make me sign exclusivity contracts! No way!