How is testing implemented in your organization? If there is a seperate test team that covers the functional tests, then the developer should focus on his units, less on the functional flows throughout the application.
If there is no seperate test team, the developer should also cover all functionality.
So, hard to tell without knowing your organization.
Ideally, functionality testing should be done by independent software testers rather than the developers themselves.
Testers are focused on testing the system requirements/functionality and hence can do a better functionality testing. This point becomes more stressed in black box testing. Testers can do even better in negative functional testing.