Hierarchical sections would go a long way toward solving a number of problems related to book structure, from automating running heads to assembling books from separate parts.
A simple example is running heads. Say I have a book divided into several parts, each containing several chapters. Say I want the left page running head to say "Part 1" and the right one, "Chapter 3". As it stands now, I have to create a separate master page for each Part and manually enter its title (the right page's running head can use the <Section Name> field to insert the chapter names automatically).
With hierarchical sections, I would only need a single master page for all Parts; its left running head would be a field, <Section Name level 0> and its right running head, <Section Name level 1>, or something similar.
Please consider implementing this.