Yes, they are useful. I use them in the German Ghibli Wiki nearly on all pages.
Adding categories. You really don't need to add categories on each pages. When you want to rename the categorie, you just need to edit the template. It will rename the categorie on all pages used that template.
Quick writing. You know many people don't know what to write about. So they can use template by fill up the template fields.
If this community grows, changes will coming. It is more comfortable to edit templates instead of editing all pages.
Can you categorize images? I know that is your strong point and it could be something easy for you to do. Just one request. Can you put images related to ponyo in categories such as "Ponyo Images" and Spirited Away images in "Spirited Away images"? Thanks. That would help a lot.