The BDC is a database/web service abstraction layer that provides a unified api.
It claims to be of general use in Sharepoint.
I can see little benefit to it if you don’t want google like searches of your application.
The big problem is that you need to be very careful or the search mechanism will invalidate any access controls that you have.
The BDC is excessively complex for what it provides. The price in it’s complexity (as is generally the problem with sharepoint) exceeds the effort required to roll it yourself…