So, of the three major hero teams, the Dark Watch seems to be the least optimized. The Freedom Five build on each other and have great draw, tanking and deck control, even with a little healing, and they have several options for huge damage turns. The Prime Wardens may lack deck control, but they put out enough cards and healing that they can weather all but the worst of storms and keep chugging along, and have several options from retrieving things from the trash for re-use, or after destruction.
Both teams have a lot of synergy and are really strong, especially once they're fully set-up.
The Dark Watch, as it exists now, seems to have a lot of trouble gaining traction. Now, granted, they are still missing one member, who for all we know will pull the whole team together and put them on par with the F5 and PW.
So the question is: What can Harpy bring to the table to solidify the team?
Card draw? Healing? Out-of-turn powers or card plays? Some other mechanic that no other team offers?
Edit: I suppose another way to look at it might be which hero do you use as a fifth with the team currently?