I plan to link to this from the task graph (e.g., View → Search Subtasks or similar, in a dropdown on the "Task Graph" element) as a way to let us bail out if tasks have 300 subtasks and send the user to a big query result list. That'll give us more flexibility to tailor the UI for reasonable numbers of tasks.
There's no UI for this unless you specify a query yourself, so the only ways to get to it are:
- Manually put ?parentIDs=... into the URI.
- Use the API.
- Future link from task graphs.
It doesn't seem too useful to me on its own, outside of the context of links from tasks.