A lack of tanks and healers is a problem in every MMO in history, and most MMOs do make you play dungeon content to continue the plot. Adding 2 new DPS classes wasn't a great idea - especially because I know for a fact quite a few Dark Knights only rolled dark knight because they wanted an epic sword, and Samurai is the new sword hotness and god-tier DPS class to boot; but even if they'd added a new tank and healer role DPS queues are always going to be bad, simply because most players don't like the responsibility or playstyle that comes with those roles.