This is sadly something that will never change, unless they remove the holy trinity or find the solution to why people prefer DPS over Tanking or Healing, untill now no MMORPG with a trinity has solved this issue. There are MMORPG's where classes can play more then 1 role, even there we see players prefering to play as a DPS over an Healer or Tank. The issue is not something related to the class, but the role.

Giving bonus experience to DPS players, would only encourage Tanks and Healers to take it easy and play a DPS. Why should I as a Healer join a group, deal damage and heal? When I can easily go as DPS, deal damage and get extra experience. To many people see the bonus rewards that Tanks and Healers get as only reward, they dont realise that the DPS are also getting rewarded, their reward is a faster queue. I am leveling my DPS classes right now and I understand how annoying it can be wait 25-35 minutes to enter a instance, but aslong as the majority plays DPS, this will not change.

DPS role is already viewed as the role with the least responsibility, the role that people pick when they want to relax and experience this game without any stress. Now imagine what kind of message this would send to the Tanks and Healers, if they find out that DPS players also get rewarded more experience when they do a dungeon. What is next? DPS players need to wait longer in a queue when farming tomestones, so lets give them bonus tomestones?

This suggestion would only encourage Tanks and Healers to play DPS and will only result in longer queue time for DPS players.