I am also using Class::DBI (although with MySQL) and I had to add a primary key to all my "join" tables to satisfy Class::DBI's requirements.

This modification of the schema simply to be able to use a module bothered me at first, until a recent discussion on the Class::DBI mailing list in which Ovid made pretty much the same comment as in the post above.

My database-fu is not very strong but I can say that I have experienced no problems as a result of this change and I have had no problems with Class::DBI.

my $0.02