Toby, the only advantage to watermarked content is that all images (and videos) do get out in the wild, and having your name on it always helps. That is why the programs do it (amoungst other reasons), knowing of course that they benefit from the type in traffic that you will never see...
If you intend to watermark content, make sure you are contractually allowed to do it. Some programs are very paranoid about it, and some content providers are not very happy about it either.
Alex