In my opinion, these are the scopes you need to consider:
- Global (across multiple sites)
- Site (across an entire site)
- Pillar/Section (used across an entire subtree of a site)
- Page (Used only on this page)
Using only one location does not allow you to scope your security appropriately for your authors. If you have a single global folder, that means giving authors from all sites and pages access to a single repository where they can potentially edit content from other sites. This is a big issue with multi-tenant and multi-site (though in some cases is actually helpful for shared content across sites).
The page-specific content allows your authors to maintain their own little 'bucket' of scope that other users can't select for re-use. Some content governance folks might not like the idea of private content, but I haven't met an author yet who didn't have the "but this is special just for me" need.
Section/pillar scope is useful when working with large sites that tend to divide their IA by department, business, or brand. You will often have different teams managing these sub-trees and often different roles and permissions. Their shared content is not the same as the site-wide header that might be in use across the whole site.
If you can provide those 4 levels of datasource scopes, you should be able to cover most scenarios for content management governance.