I try to include an alt tag with every image. I really try to make the tag appropriate to the actual image, but if possible include the keywords of the page. I don't think it is necessarily a good idea to use the keywords if it has nothing to do with the photo.
I do generate free traffic from Google from the images, so it is a good idea.
The only thing that it would potentially be useful for is Images, and for helping blind people naviagate your site. As far as SERP ranking goes, on page tags like that do next to nothing... at best.
