Guild Wars - State of the Game @ Official Site

A new State of the Game article at the official Guild Wars sites takes a closer look at secondary professions.

Guild Wars characters are based on one simple but ingenious system—each character may use one primary profession and one secondary profession. Although it may seem like players use skills mainly from their primary professions, the secondary profession can actually have an enormous impact on the success and usefulness of a character, especially within PvP play.