The composer layer is only needed when building organized interfaces. The sophistication of the composer varies heavily based purpose.
Many games have chat screens, HUDs, and other auxiliary interface items that do not require sophisticated layout mechanisms. On the other hand, business applications start requiring grid-based layout and other interesting schematics. When building documentation, other layout systems are directed toward presenting paragraphs, words, and media.
One could imagine that a developer's IDE requires these multiple strategies: code documents, debugging HUDs, and docking panels. Most systems are optimized for one or two, but never 3 strategies. If composers are pluggable, we can quickly adapt areas of our screen for each discipline.