One question first:

It allows users of libtaskmanager which are solely interested in window data (e.g. the Pager) to skip the unnecessary complexity of TasksModel, while still sharing the windowing monitoring with users of TasksModel.

What's your longer term plan?
To export WindowTasksModel directly? You'd still need to go via TaskFilterProxyModel, and for the pager you'd need AbstractTasksModelIface too (for setVirtualDesktop)
...at which point you're pretty much back at using half of TasksModel anyway?

To export WindowTasksModel directly?

It's already exported ...? See the change to CMakeLists.txt and the TASKMANAGER_EXPORT macro.

You'd still need to go via TaskFilterProxyModel

Yes, TaskFilterProxyModel is also useful with some changes (see D2380).

and for the pager you'd need AbstractTasksModelIface too (for setVirtualDesktop)

Indeed, libtaskmanager also installs the abstracttasksmodeliface.h header.

...at which point you're pretty much back at using half of TasksModel anyway?

That's addressed in the review request description: The Pager doesn't care about startup notifications, or launchers, or TasksModel's sort modes, or grouping, or has a need for a ConcatenateTasksProxyModel, ... all it wants to share with TasksModel is the window tasks sub-model (and from libtaskmanager it also wants to use TaskFilterProxyModel).