The Ubuntu package for digikam does NOT depend on Konqueror or Dolphin, but it instead recommends kipi-plugins, which in turn recommends konqueror und suggests kmail and so on...
Since Ubuntu by default installs recommended packages (but afaik not suggested packages) konqueror is "pulled in".
So to install digikam with the kipi-plugins (and exiv2), but without Konqueror and Dolphin one could use the following command:
This declares a strong, but not absolute, dependency.
Suggests
This is used to declare that one package may be more useful with one or more others.
I think that this fine-grained dependency tracking allows Debian (and derivatives) to strike a good compromise between simplicity and configurability in terms of dependency management.