I don't get why people get so protective with their photos. It's art, I would be pleased if people shared my photos (credit or not). And even if they claimed them as theirs, does it really matter in the end? In the end you know it's not their art, and they have no way of doing something similar or proving that it's theirs, no that it matters, you KNOW it's your art.
That's how I see it. That's not to say I don't have a watermark. I think, like Geoff said, a nice, elegant watermark adds to the picture if done right.