Portia doesn't get much attention of her own these days, but back in the 19th century, when women writers were just starting to enter the field of Shakespeare criticism, quite a few female authors rallied around her as a symbol of female perfection, a vital role model for English women, and proof of Shakespeare's understanding of and affection for women.
One 1888 essay contest in a magazine for young women, The Girl's Own Paper, was centered on 'My Favourite Heroine from Shakespeare', and reveals a great deal about what girls and women thought of Portia at the time. The Girl’s Own Paper received a record high number of responses for the competition--more than a third of which were devoted to Portia. (see Girl’s Own Paper, 10 March 1888). The girls praised Portia’s balance of what ‘is excellent in a man, with the gentle charm which is so essential in a woman’ (same GOP issue, p. 181), echoing other women writers and critics like Anna Jameson and Mary Cowden Clarke who similarly celebrated Portia's strength and femininity.
In writing about Portia as a role model, Anna Jameson revealed her belief that while Portia is real in the sense that Shakespeare wrote her as a fleshed-out, believable character, the strictures of Victorian life make it impossible for a woman as perfect and balanced as Portia to exist and thrive in the real world.
She is in herself a piece of reality, in whose possible existence we have no doubt: and yet a human being, in whom the moral, intellectual, and sentient faculties, should be so exquisitely blended and proportioned to each other; and these again, in harmony with all outward aspects and influences, probably never existed—certainly could not now exist. (Jameson, Characteristics of Women. New York: Saunders and Otley,1837, p. 57-58.)
Still today, though, we see her as a woman who defied the conventions of her time to defend her husband's friend in court. She has faults by today's standards (I'd be worried if I had a female friend who got married to someone in great debt and immediately signed over control of her entire life savings to him!), but I would certainly not accuse her of weakness.
A comment from Samuel Johnson's introduction to his edition(1765?) comes to mind: "Shakespeare has no heroes; his scenes are occupied only by men, who act and speak as the reader thinks that he should himself have spoken or acted on the same occasion." One example is the scene in ROMEO AND JULIET where Romeo tries to stop the duel between Tybalt and Mercutio. The result is that Romeo disables Mercutio's sword and Tybalt gives Mercutio a fatal wound. Therefore, we might consult experts in law enforcement regarding what Romeo might have done instead while we regard his intention as reasonable.