Using DBI was just an example. I had a script that used Win32::ODBC. I ported it over to UNIX because I wanted to add support for Oracle, not just Access databases. It was nice to add the functionality without breaking the old stuff -- or having to rewrite it.
There are times where it would be nice to either use or not use, was my point. For example, using a package if it exists, exiting more gracefully if not. It has been an issue with some of my perl scripts (until now).