In the days when I only used film, clients would occasionally ask for negatives. The answer was simple: no. As the creator of the image, I retain copyrights to my work. My work, at that time, was in the form of a negative. So, the explanation was typically well-taken & understood.
Digital "negatives" or files, are a different animal. When I am shooting for a client (and most other times), I shoot RAW. I do, on occasion, have clients who ask for the RAW files. My answer remains the same: no.
I think that most people think that RAW simply means "unedited", but it's more than that. I came across an article that explains it so much better than I have ever been able to do. Click on the title to read the article on The Phoblographer: This is Why I Won't Give You My RAW Files.