Addendum: subpages. subpages are enabled in the project (Commons) and user namespaces. This means when you create User:page/foo or Commons:page/foo, it will have an automatic 'parent link' to Commons:page. And you can also use Special:Prefixindex to find all the children pages of Commons:page.
Subpages have not been enabled in the template namespace. This is why template:PD-old/es doesn't contain an automatic parent link. If subpages are not enabled, then the slash is nothing special, it is just another character.
AFAIK, besides the fact that templates can take parameters, pages can be transcluded in the same way as templates, the only difference being that they need the namespace prefix. So
{{Commons:Welcome}}
is the same as
{{Welcome}} if Template:Welcome has the same information as Commons:Welcome. So if you don't need to use parameters, then using pages in the Commons namespace has the added advantage of automatic parent links.
Just wondering if we can incorporate this into a new design somehow......
cheers Brianna