BTW the use case for a subsite-wide level of visibility -- as opposed to an application level such as what you're describing for ecommerce -- is this: a subsite created for management of a large clinical trial (one of many different projects each of which has its own subsite) deals with "enrollment issues", "data collection issues", etc etc. These categories pertain to discussions that would occur in forums, events in calendars, and descriptions in xowiki pages. These categories do not
pertain to other subsites in the same site organized for other projects/purposes, such as a subsite used by senior management of the clinical research organization for personnel issues, etc; nor do they match categories used in the public-facing stuff in the main site.
If only site-wide and application-level visibility is supported, then the category trees would need to be duplicated for each of the applications -- forums, calendars, xowikis, etc within the subsite -- which is error-prone and unnecessary. The subsite-wide level allows all these applications to share the categories without having them leak out to other subsites or the main site.
Probably this was clear, but I thought I'd make it explicit.