The role of plunge in the opener gets revisited every so often and is met with heated debate.

If you look at the openers of the top performing DRKs, you'll find that it's situational. For the typical boss which starts to move towards you after you tag it with unmend, plunge won't save you much travel time because you're effectively plunging from a closer distance. The time that you save is offset by the animation lock of the skill. As a result, it often gets used later in the opener, as there are higher potency oGCDs that you may wish to use first.

If the boss is immobile, or if the boss starts the fight animation locked due to a cast (A5S), you're using plunge enough distance to offset the animation lock, so it does save you travel time. In such cases, you will see plunge being used after unmend.