Of the layers of the software architecture, the business logic layer is the one that has long-term implications and strategic value to a business enterprise. User Interfaces (UIs) come and go with technological changes but the value of the business logic is much more constant. In many cases the same core business logic implementations successfully serve their user community over many years through first block mode, then GUI, and now Web front ends. It is the inherent value of the business logic that makes this possible.
If we accept that the business logic is the long-term strategic investment then obviously that is where we should focus our efforts to make them robust, high quality and maintainable. A significant part of this focus is testing. Testing tools typically address testing via the UI. While UIs must be tested for apps to be usable it is important to note that if testing is performed strictly via UIs we are restricted to testing logic paths that are supported by and invoked by actions available in the UI software.
The net effect is that the underlying business logic is not fully tested and therefore cannot be thought of as completely tested. Of course, this has potential downstream effects in the maintenance cycle of the system. It also does not support component based development fully or even partially. The business logic 'components' (i.e. black boxes) are not 'certified' to be functioning as per their 'interface' or API implemented as import/export views.