Primary keys: composite and singular

In general when I design tables I always include a single unique field, even if it there is already a unique combination of columns available to create a primary key. This field is usually either a UUID (if application-generated) or a sequence (if database-generated).

Some questions:

• are there any downsides to this approach (apart from storage)?


• the sole exception to this rule is (for me) the join table that exists only to join other tables. Is there an argument to be made that a join table should also have a single field acting as primary key?