The latest edition of the State of the Game article series has been posted on the Guild Wars official site. This article covers the usefulness of the secondary profession.
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.
Even though putting together a Skill Bar with only Ranger skills isn't too difficult, Rangers often use a secondary profession. Ranger/Mesmers, for example, were dominant before Distortion was updated. Rangers packed Distortion so they could run flags or split. Distortion was a perfect skill for Rangers because the excellent Energy management provided by Expertise meant they could afford to pay for Distortion's Energy loss.
Nowadays, most Rangers in GvG run as a Ranger/Monk. This is due, in large part, to the skill Mending Touch. With its 5 Energy, three-quarter second cast time, and 6 second recharge time, Rangers using this skill can almost certainly counteract the effects of any Blinding Surge Elementalist or other Condition hate they might run into.
To a lesser extent, Rangers also use Purge Signet. Purge removes all Hexes and Conditions on a target, and it easily fits onto a Ranger's Skill Bar. Purge Signet is much more common when the metagame goes heavy into Hexes. These two skills affect Ranger usage immensely; without them, we would see much less splitting and more Hexes or more Hex removal on non-Monk characters.
Read more here.