1. Google may cache all images in all emails sent to gmail.com instantly and regardless of the existence of the address. This would remove the possibility for marketers to check user timestamp, remove user data from request and hide user email existence.
2. Google does _not_ need to save each image from each unique URL separately, all they need to do is fetch each image and check against an already existing (mega)array of images they've fetched. This greatly reduces storage needed, but doesn't do much for the bandwidth requirement, but they won't care about bandwidth in all their Googleness.
3. The single most important aspect of this change has been omitted in the article, and in your comment: This change completely eliminates the risk of CSRF attacks by spammers and the likes. CSRF attacks are still number 8 on OWASPs list of top 10 attacks.
My three cents ;)