i fed ds till 18 months, but he had cows milk sometimes, and we slowly reduced down to night feed, and then cut.
i don't know if there is any benefit, my son as chronic asthma, bad sinuses is prone to ear infections, seems to always have a cold despite exclusive bf till 12 months and combined bf & cows milk after that.

one thing i do know is that i demand fed, and weaned slowly, one feed at a time and I did not get engourged, mastitis or feel depressed or guilty, it just felt right, it benefited me, probably just as much as it benefited him. if your not ready and she is content keep going, there is no shame in it, and if you check out the list of ingredients for breast milk v formula kelly posted you will see the nutritional benefit.