As a Rapid Board user I would like to be able to see sub-tasks of top level issue types if they are not included in the filter my rapid board is based on.

The main problem being addressed here is that it is not possible to have a filter which includes user stories, but not their tasks and still have the tasks shown on the rapid board.
A actual example of when this is required is the following. A company has X number of development teams that each have a rapid board. Each user story has a 'team' field which is tagged with the team currently working on the user story. The filter the rapid board is based could look something like 'team = "teamname"'. This filter would result in X amount of user stories being listed. With the current solution it isn't possible to list the subtasks of those user stories.

With the planning board this was not an issue as it would filter on fixVersion, which would include the subtasks.

A workaround for this would be to simply tag the subtasks with the same team as the parent. While this is entirely possible, the hierarchy would break as soon as the team is changed on the parent.

Alternatively the JQL could be more advanced and support queries such as "issuetype = Task AND parent.team='teamname'"