As a developer it would be awesome to have shared tabs on a custom type with the purpose of removing annoying maintenance.
Fx I have a Landing Page, Blog Page and a Legal page (policies and stuff) - Since all of them are pages I want them to have similar navigation options and SEO options. At the moment I have to create a new tab on each page and copy/paste the navigation and SEO structure into their respective tabs. If I want to add more to my SEO tab later on I would have to also update the other custom types.
If I instead had the option of creating a shared tab I could update them all at once <-- nice.
This is a feature I have experienced in multiple other CMS's and It is very appreciated
I'm aware I could achieve this with slices, but I want to keep slices to be focused on content and not on page options.