I've seen this subject discussed in numerous photography and stock photo site forums. People on both sides of the question have their valid reasons why they do or do not prefer watermarks. I've read forum threads which highlight concerns as to whether the watermark is too intrusive, not prominent enough, multiples are too many and plenty of other remarks.
I'm of the belief that if you leave an opening, someone will take advantage of it whether realizing or not that it is stealing. If you block (watermark) that opening, you may slow down or make some individuals think twice or it may require too much work that it may just keep someone from even trying. So I'm of the mind to watermark my images knowing that if individuals really want to steal my work they will find a way but I'll make them work for it. Contact me via the blog comments if you would like an image I post here and I'd be glad to work with you in providing what you need. I will not publish the comment you submit so it will be a private correspondence.
What is your opinion on watermarking?