Unique or Primary Key?

This table is man-in-the-middle of a many-to-many relationship:

CREATE TABLE cv_entries (
        subscriber INTEGER NOT NULL
                REFERENCES subscribers
                        ON DELETE CASCADE
                        ON UPDATE CASCADE,
        entry_type INTEGER NOT NULL
            REFERENCES cv_entry_types
                        ON DELETE CASCADE
                        ON UPDATE CASCADE,
        ordinal INTEGER,
    value1 TEXT,
    value2 TEXT,
    minimum_trust SMALLINT,
    UNIQUE(subscriber, entry_type, ordinal)
);

I used a unique index here because I couldn't see any reason for a
Primary Key - this table will always be searched on either the
subscriber or entry_type index.

Was I wrong?  Should this be a Primary Key?

--
Bruce

Bitterly it mathinketh me, that I spent mine wholle lyf in the lists
against the ignorant.
        -- Roger Bacon, "Doctor Mirabilis"