While it is true that FrameMaker excels at the creation of printed content and PDF files, there is absolutely no reason that it should be limited to the creation of chapter-based and book-centric content. FrameMaker works with files, you get to decide what those files represent .. books, chapters, sections, topics, or snippets. For years, people have been using FrameMaker along with tools like Mif2Go, WebWorks, and others, to create both PDF/print output as well as numerous online formats. In those workflows, people generally authored in unstructured, chapter-based files.
With the growing popularity of XML (and DITA in particular), FrameMaker can still be used to author content in a chapter-oriented paradigm, but it can also be used to author individual topics just as easily as any other XML editor. Using DITA maps to organize the topic files (which may represent books, chapters, or other collections). These map and topic can then be passed to external processing tools (like the DITA Open Toolkit) to be generated into specific deliverable formats, and can also be processed within FrameMaker to be assembled into books.
You might suggest that authoring in FrameMaker forces people to think in a book-centric manner because of the print format of the WYSIWYG authoring view. Although this may be the way you’ve seen FrameMaker used, this is completely up to you as the author to format the authoring view however it makes sense for your workflow and writers. When opening an XML file, you select a structure application to use, which applies a template which defined the way the content is rendered on screen. You can set up this structure application with a print-focused layout or you can choose to use fonts and a layout that looks very simple.
One example of this is found in the default DITA-FMx 1.1 Topic application which has no indents, uses Verdana as the font throughout and looks more like a web page than a printed book. If you choose to create a PDF from the DITA map and use the default Book application, the print formatting is applied. But while authoring, using the Topic application, you are encouraged to focus on the content rather than the formatting since it’s likely that you are creating deliverables for multiple output formats, of which print (PDF) is only one.
Just because FrameMaker has traditionally been the standard tool for authoring long documents doesn’t mean that it can’t be used for topic-based authoring as well. It is a very flexible and customizable authoring tool, which should be evaluated carefully with other tools when selecting an appropriate XML editor.