The background was created for pending dialogs:
It takes up a small area as in bounded by a border. It shows that the dialog is in a semi-disabled state, waiting on an asynchronous process.
However it should not be used as a generic placeholder for loading content, as it is currently being used on the notifications page, and RC filters:
Here the texture is used as a foreground, not a background, has no border leading to sharp edges, and is of a fairly arbitrary size.