I think that there are couple of ways to approach the statement. On one hand, I don't see Emilia or Desdemona lack importance. They are of vital significance. Desdemona's position is a contrast to Othello. Whereas Othello is increasingly atrophying and becoming more of a victim to his insecurity and doubt, Desdemona is the embodiment of love and how, at times, love goes silent and unacknowledged. Emilia is not cast as someone who lacks power or importance, as she is critically important for helping to bring down Iago. I don't see Shakespeare as using these women as decorative and as "window dressing." He includes them in the most significant of relationships and most powerful of plot developments. Finally, I don't think that they "offset" the tragedy in the drama. They are a part of it. Desdemona's forlorn state, one in which she leaves with Othello only to find that that she has no one to which to turn when Othello begins to fragment is tragic. Emilia's constant support of Iago until a point where she can no longer reclaim whatever goodness might be there is part of the tragic condition of all the characters. I don't think they offset the tragedy of the drama as much as they are a part of it.