It's like this in any MMO with queued dungeon group finder tools. Look at the player breakdown by class in any game, and it's 15-20% tank, 15-20% heals, and 60-70% DPS. It's the meta of tanking and heals that drives people to play DPS, not the number of available classes. Queues will be basically the same, because very few people are at "all jobs at 60" and even those that are have preferred mains they'll get to 70 first before they do new/shiny with RDM and SAM.