I have this set up in a "boilerplate" gulpfile for when I occasionally use sprite images. Occasionally being my keyword here, often times, I won't ever actually be using the sprite task.
However, I'd still like my watch task to watch the directory, just in case. But if the src directory doesn't have any images to turn into a sprite, it looks like it's not finishing and more or less killing the watch task, making it so my other tasks in watch never fire.
If there are images in the src directory, it generates fine and the rest of the watch tasks fire fine. It really just seems to be if there's nothing for the sprite task to do.