One use case that keeps coming up in my mind is the fact that sub-projects imply the parent project, so instead of having several similar tags on a task you can just tag with a specific sub-project and let that imply all parent tags. Specifically I think this makes sense in cases where we have herald rules set up to auto-add one specific project to all tasks that get tagged with a related project. All of those herald rules should be replaced by sub-projects, IMO.

We've started using milestones in the Scap sprint project and it's working nicely so far.

See https://www.mediawiki.org/wiki/Team_Practices_Group/Proposal_for_work_categorization