[JonathanLamb]

I'd just like to toss a vote in the hat for this functionality. To be honest, I was pulling my hair out trying to find this when first using OmniGraffle as it seemed like such a bafflingly obvious feature! I actually expected Stencils to act in this way for the sake of sharing object libraries within an organization.

Ideally for me, we would be able to place an external graffle as a sub-graph or stencil within a new document. Each placed instance inherits from the original document, but deltas are saved in the current graffle (with a contextual menu and/or inspector command to clear "local" deltas, and "bake" selected objects to remove inheritance).

This would allow one or more people on a development team to share a master library of re-usable components across many documents, while accommodating tweaks and content "branches" for testing revisions or moving between projects with unique requirements.

For wireframing, beyond maintaining objects like headers, footers, logos, etc, we could then for example stub in "gray box" content/navigation areas, and iterate individually, delegate to different content owners, etc. (not to mention maintain consistency across company or project documents).