Even if the image is decorative, it is still describing the contents of the image to visually impaired users. Here's more from Google:
https://support.google.com/webmasters/answer/114016?hl=en
From Google:
"The alt attribute is used to describe the contents of an image file. It's important for several reasons:
It provides Google with useful information about the subject matter of the image. We use this information to help determine the best image to return for a user's query.
Many people-for example, users with visual impairments, or people using screen readers or who have low-bandwidth connections—may not be able to see images on web pages. Descriptive alt text provides these users with important information."
The image's decorative value is for the user to judge, it's about providing the full story and experience to all users not some.
