For example, some Django projects implement only a REST interface. Since those projects don't rely on HTML templates, they don't likely incur any runtime costs

For example, some Django projects implement only a REST interface. Since those projects don't rely on HTML templates, they don't likely incur any runtime costs from Django's template modules. In effect, you can choose which parts of the Django project you import. The parts you don't import aren't likely to get in your way. I think this is generally true for modular software.