Sometimes or rarely, latest updates of WP, themes, and plugins have small issues/bugs which might break the functionality of a site.
If auto update run weekly, the update might trigger some errors if the update itself is buggy. To prevent this, update monthly option could add some more time to plugin/theme developers to fix it.
ManageWP has no immediate plan to offer auto update monthly feature.
BlogVault already has this option.
WPMUDEV allows either everyday or every week, but I’d like to see monthly auto update feature, too.