The ability to link issues in a parent child relationships would be a very powerful feature.

In my opinion, this feature would be targeted more to project managers and product owners and anyone who would be involved in the creation of product backlog, managing the prioritization the delivery of that backlog and reporting against enterprise projects.

Parent child links would allow project managers and product owners to elaborate a product backlog and represent it as a hierarchy of successively smaller and more detailed backlog items.  For instance a backlog could start from a “root“ issue of Type: Project which could be broken down as one or more Features, each with there own Epics which would be further broken down into Stories and their Subtasks.

Creating a backlog as a hierarchy of linked issues would be useful for managing execution and reporting against project delivery.  Automated workflows could be written that would traverse an entire backlog or just a single branch of that backlog to apply logic such as updating parent issues when child issues change.  A hierarchy of linked issues could also form the basis of customized reports that could communicate, among other things, overall project status or feature completion status.