Excluding healers and arcanes, there are very few roles in DDO that cannot be accomplished while also bringing DPS (although healers can also do DPS). Consequently, if your specialist neglects DPS to focus on their primary role it is likely suboptimal. Obviously certain builds like tanks and bards won't have the DPS that a barb or kensei fighter does, but they can still be built to accomplish their primary role (tanking/buffing) AND contribute meaningful DPS.
You used rogues as an example, a trap rogue is almost useless in most of the content in DDO. For the investment of 3 feats and some build points in STR, you can still do all the traps in the game and contribute significant DPS.
Yes there are exceptions, spellsinger bards come to mind, but in those cases it is pretty clear that the benfits they provide by neglecting DPS are significant.