She had a good part to play in Disaster, she was shown to be inept as a command officer, but she was supposed to be. Also the episode with the cosmic string where she loses her mental abilities.
Plus, I'd like to think that after Deanna hooked up with Worf, that she was "well used" on a nightly basis.
I just watched "The Loss" last night (cosmic string). She is really effective in that episode, having to deal with a sudden and painful disability. Marina Sirtis can act, and when they gave her good opportunity to do so, she did very well.