Shakespeare uses the women in Richard II strategically. Although they may be considered minor characters, their importance (as with anything Shakespeare included in his plays) is crucial to the development of the plot and characterization, and the revelation the story's themes.
The Duchess of Gloucester adds the touch of human emotion to the play. As a widow, she shows the depth of her love for her deceased husband who she misses terribly. She also serves the purpose of showing how a family experiences the loss of a loved one, especially (in this case) a husband, which is much different than the loss one experiences for a friend. Her love will also be held up as a source of comparison with regard to the loss other characters experience in the play. And in her sorrow, her character repeatedly helps to establish and feed the mood of the story.
The Duchess of York also exemplifies a kind of love: in this case, parental love. The complication to this love is that her son is complicit in the plot to kill Bolingbroke. This places the Duchess at odds with her husband; she cares less for politics and more for her child, and the Duke cannot afford to place his child above his politics.
As with Richard III, the women in Richard II occupy minor places within the society (as was common for the time), and the men elevate king and country beyond all else (a necessity for survival at the time). However, this propensity of the men of the time to place politics above all else shows them as hardened, lacking a human dimension, while the women in the play elevate the human emotion of love for another to provide balance and a clearer snapshot of Elizabethan society.
Queen Isabel shows yet another connection between a man and a woman, especially when compared with the Duchess of York; whereas the Duchess defies her husband for her son's sake, the Queen supports Richard in all things. Her character also helps convey the dark mood presented by the Duchess of Gloucester at the play's outset, and it is through the Queen that Shakespeare provides the audience with foreshadowing that some horrible fate waits on the horizon [Richard's murder].
Minor parts they may have had from a historical standpoint, but in terms of the play, the women's roles are nothing if not necessary to set the mood, define other characters by their presence, and move the plot along thereby making the story's themes that much more powerful.