The limitation of only having two levels in a cascade select field makes it difficult to create screens which adapts to the users selections. Removing this limitation and allowing multiple levels makes it easier to create "selection adaptive" screens.
Another way the same functionality can be achieved is by extending the context for fields from Global/Project/Issue to include selections from a select list/radio buttons (or even a user picker?). This would really make screens work a lot better!
Extending the field context would give users of JIRA a whole range of new possibilities related to customized screens, and would also extend the possible use cases for JIRA.