In addition, If you need to gut one part of your system for another (say, DBD::CSV -> DBD::MySQL -> DBD::Oracle), I would much rather be able to change (basically) my connect string, test, test, and test, rather than have to rewrite the guts of each of these.

Sometimes the overhead is due to bolting a consistant interface on your module, so that it can replace / be replaced by something less / more appropriate.

Good question, though.