I haven't done many of the "optional" dungeons, since I am holding off until my husband hits those levels as a new player, but I can say that for the dungeons that I have done, the exp has been great. I just did Toto-Rak, and I got about 48000 exp when I synced to lvl 27. I have done Stone Vigil recently as well, and I didn't monitor how much exp I got, but I do recall that the mobs were consistently giving me between 1,000 and 2,000 exp each, and those were just the trash mobs. I don't know if the exp keep scaling up at the higher level dungeons, but I would hardly call them worthless. And for the drops, I believe you can sell alot of the loot on the market for a decent price.

I understand that the DF can be a chore when going as a DPS, but if you are a part of a good Free Company, I would suggest that you look for someone in your FC that needs help with lower level dungeons as they are needed. You will be getting easy experience as well as helping out other players in need.