Unless I'm missing something, wouldn't the child dependency be enough to prevent the unit from being dropped into another library or app? That's a good point you bring up about knowing where to "test drive" the changes from, though usually on the apps I've worked on, they've been small enough that the relevant integration test could be found without much detective work.
I guess I haven't been involved in too many 2-3 year monolithic projects. Maybe that's when a stricter symmetrical unit test policy makes the most sense.
What other levels of tests do you end up running besides your unit tests? Do you have any integrated unit tests? Functional tests? End to end tests?
