The interplay of each character's needs -- some selfish and foolish, some practical and kind -- is about as true-to-life as one can get in the tiny microcosm of a film.
Among other things, I love the irony of Aggie's giving her daughter the usual "it's not all romance, it's about living together and working together" speeches, while she's also angling for a big wedding which will make up (she thinks) for her disappointment at feeling her husband was bribed into marrying her, and his perceived stinginess over the years.
That fancy wedding would destroy her husband's goal of independent work (which could also cost her money, and make the daily life of marriage even harder), and wreak financial havoc with the young couple who can't afford to be best man and matron of honor if the wedding is fancy. It would deprive her child of a wedding trip that could start her marriage off well, and deprive the couple who are expecting a baby of getting the car they need.
So she's looking for the big romantic illusion, while messing up several couples' ability to deal with some of those everyday challenges of marriage.
And I love the fact that Chayevsky doesn't hit us over the head with that irony when Aggie finally comes to her senses, makes peace with her husband, and accepts that the expensive romantic symbolism would not be some sort of payback.
She doesn't apologize, or rehash the whole plot to show us that she has learned her lesson, or say "what was I thinking -- this IS the everyday work of marriage I was telling The Girl about!" That would be out of character (and would be likely to be in a movie that wasn't as well written, complete with "listen, folks, here's the moral of the story" music behind it).
She just pays for the cab medallion, and cracks up laughing at her husband's happy surprise.