I think that's precisely why this testing insight isn't more commonly used. In reality, it's not about unit vs functional, it seems to be about scope. E.g. If you have behavior that can't be tested without doing a full manual integration test, then that's where you need to start. Automate it and start to push back the layers until you get to functional, until you get to unit.

I've been experimenting with applying code coverage to integration test since it gives testers insight into whether they are testing the code at hand vs internals of other integrated systems (this difficulty is likely what keeps many from trying such testing).

But yes, these methods take a lot of time that we don't have.

The other approach I've been leaning towards is static code-flow analysis, which can also be difficult (esp. in distributed web applications).