GCE images are built more than once per release currently, primarily to to improve the GCE-specific integration code and CLI tools or adapt to changes in the underlying environment and feature set. Occasionally there's something else, like the performance-driven backports change Himanshu mentioned on this list recently. So the CD frequency is only good as a minimum, not a maximum.
That said, the more of the build process that Debian can automate, the better. I'd also suggest that careful thought be given to testing; right now we have some internal tests that we run for the GCE images, but pre-publication testing is a generally useful pattern across all environments.