Using <navtitle> in DITA maps allows us to use more concise and appropriate titles in the bookmarks and in the document Table of Contents in PDF/Print output.
Unfortunately this does not currently work for "chunked" outputs like web/html/chm.
This was raised as a support case, where the conclusion from PTC was: "The developers have looked into the behaviour in more depth and the approach the stylesheet writer took will not support the TOC for chunked output. This is how it was implemented to work, the output is what is expected."
So currently the processing does not fulfill the expectations of the DITA specification: "The navigation title (<navtitle>) element is one of a set of alternate titles that can be included inside the <titlealts> element. This navigation title may differ from the first level heading that shows in the main browser window. Use <navtitle> when the actual title of the topic isn't appropriate for use in a table of contents, navigation pane, or online content (for example, because the actual title is too long)."