It can still be interesting to know when an image was first seen, even if the link is now dead. Also sometimes a page only moved URLs or changed its site structure and the picture somehow didn't get indexed at the new location, so the dead URL still helps in tracking things down (rarely, but still). Sometimes you can find something on the Internet Archive.

Also in my experience Bing has a similar tendency of never letting go of things once it has indexed them, even when the result has been dead for quite a while. On the one hand this can be frustrating, but sometimes having just Bing's image preview is still better than nothing at all.