The problem with DF ques for dungeons is the 4 person party, that must consist of one tank, one healer, and two dps. If you look at this slide from the Census 2014 (
http://imgur.com/a/pEzlr#8 ) that shows the role distribution among the player base, you can see that DPS are 38%, to 10% tank, and 7% healer. So DPS are nearly 4 times as prevalent as tanks, and more than 5 times more prevalent than healers. Adding more tank jobs and healer jobs, won't necessarily mean more people will play those roles, though some might be enticed to switch from DPS to those roles if they are exciting enough. The only sure fix would be a six man party for dungeons with 4 DPS, one tank, one healer or a combination of DPS and Support (should they ever be added).