Check the terms of service of Flickr. You can set the privacy settings of your photos on Flickr to private, but Flickr is a service designed to display your photographs to the general public. The reason you don't see as many watermarks and such on Flickr is the professionals who are selling their photographs use other services. It is not designed such that you can post a low resolution photo where when you click it, it brings up an option of purchasing a high resolution photograph.
Smugshot or services similar to that are the type of services that professional photographers use to sell photographs. On those services a person can view a lower resolution photo but still have the option to buy a print or a high resolution photo. The main use of Flickr with watermarked photos is for posting on Forums such as this. Watermark it to keep someone from claiming it is theirs is about the only reason to post a watermarked photo on Flickr. It's not designed as a service to display "proofs."
No reason not to watermark it to "claim" the photo. The reason you don't see it more is just because people aren't doing it there to sell photos.