The Merchant of Venice revolves around loyalty and honor. Misunderstandings abound and cultural differences lead the play in various directions.
"The importance of one's word" is relevant for Portia as she intends to fulfil her late father's wishes, despite her protestations, as she must suffer " the will of a living daughter curb'd by the will of a dead father."(I.ii.21) Filial (family) loyalty is unquestionable and Portia will do what she can to manipulate her suitors in their choice but would not even consider withdrawing from the arrangement.
Even Portia's suitors will be restricted to a life of celibacy if they choose the wrong casket. There does not seem to be any question of them reneging on their word when they leave, unfulfilled. It will be expected of them to keep their word.
To further develop this concept of one's word, Antonio promises to pay Shylock with a "pound of flesh" should he be unable to pay the loan that he takes out on Bassanio's behalf. Equally, Shylock has no intention of pardoning Antonio - "tis mine and I will have it." (IV.i.100) It is inconceivable to Shylock that he should pardon Antonio as the law decrees and the law is fixed. There is no room for compassion in the execution of the law. The Christian faith encourages compassion as Portia expounds: "It blesseth him that gives and him that takes" (IV.i.188) but, in the Jew's eyes, the law is above all else. Having given his word, Antonio is bound by that law.
Jessica, Shylock's daughter, does not, however, hold her loyalty to her father or her religion in high esteem. Her disregard is evident -"I shall be saved by my husband, he hath made me a Christian!" (III.v.19). She is prepared to risk all in the name of love, apparently and simply to hurt her father; such are her feelings towards him. She promises Shylock that she will take care when her father warns her about the Christians - "Lock up my doors" - but she is already plotting her escape with them. Jessica's opposition develops the plot as she is the reason that the audience eventually feel some sort of pity towards Shylock as he is deeply hurt by her actions.
Hence, keeping one's word filters through all the characters in The Merchant of Venice as the plot develops. Justice however must always be tempered with fairness and compassion.