One problem. Both Kami and Karavan have participated in stopping the convoys. Tagging for PvP would hamper their efforts to work cohesively to stop them. Efforts in ToT were hampered by the GvG PvP zone.Having that carry on into BB would have probably led to the convoy succeeding as there was little time in Void between the leaving of PR and the convoy success reaching Lost Valley 600 meters from the portal.
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