You might not need to use all of the systemd tools but its tools
aren't independent.
That is similar to how optional features are handled in many collections. If you use some features, they might pull in other requirements but the features themselves are optional.
For example, Ubuntu patches logind in order to use
it with upstart rather than with systemd.
systemd explicitly documented which interfaces they consider independent and which ones they don't and I linked to the document earlier. Ubuntu's use of logind is backporting + reimplementation of some interfaces using a
shim and they are moving away from it to systemd itself since the
reimplementation is lagging behind in features and functionality and the Debian move to systemd made it easier for them to follow that path.