Our company just purchased this to have a place where we can easily search and get images of our drilling equipment. Having tags is a must, but subcategories would greatly increase this tool's usefulness as a supplement to tag. Our thought was to have a category for each type of equipment, and a subcategory for each different machine. I still think this is a needed feature in a software that targets large image galleries like yours.
That said please prioritize tags those are even more important.