I'm looking for some good reasons why you would want to use a bookmap instead of a ditamap.

Obviously, you use a bookmap if you wish to "to produce your DITA topics and even whole DITA maps as the content of a formally defined book. This allows you to produce not only maps for online deliverables, but also PDFs with the same content, replete with covers, formal notices and frontmatter, and so forth." (Source: DITA Architectural Specification v1.1).

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