Should each and every table have a primary key?

Just add it, you will be sorry later when you didn't (selecting, deleting. linking, etc)

To make it future proof you really should. If you want to replicate it you'll need one. If you want to join it to another table your life (and that of the poor fools who have to maintain it next year) will be so much easier.

If you are using Hibernate its not possible to create an Entity without a primary key. This issues can create problem if you are working with an existing database which was created with plain sql/ddl scripts, and no primary key was added