OP's description is the exact reason this game needs in game parsing for everyone to see.

Most of these DPS don't even know how badly they are doing, which is quite sad and a huge detriment to every party they join. Most players tend to end up avoiding them and the poor DPS doesn't know why he isn't invited anymore, which is also a bit sad.

If there was something apparent to all, showing your DPS output, they would at least have awareness and the opportunities to have that discussion about improving their rotations would be much easier and much less defensive.