Philosopher, I beg to differ. The main thing that changes with boys is their requirement to daven shacharis with tefillin, and that they may be counted in a minyan. The boys AND the girls, are EQUALLY chayavim in fulfilling those mitzvos that are incumbent upon them. It matters not that boys have mitzvos that girls do not. Girls also have mitzvos that BOYS usually do not.
It IS a momentous occasion when a girl at 12 years and one day (a full year before her male counterpart), is required to fulfill the mitzvos. And that day SHOULD be marked in some way. Not as a seudas mitzvah(unless her father decides to make a siyum that day), because their is no seudas mitzvah for this occasion otherwise, but a simple birthday party can be made for her family and/or classmates. There is no issur in that. The issur is in people equating the bar and bas mitzvahs in such a way as to actually teach a girl the Torah laining (or haftarah)or even the brochos for an aliyah and call her up to the Torah, which of course is not halachically acceptable.
I see absolutely no halachic reason not to make a girl feel special at the time she takes the ol malchus Shamayim on herself. And it is a wonderful opportunity for her to give a D’var Torah and show that those first 6 years in yeshivah have been productive. What I DO object to, is the oneupsmanship some people feel they need to display. Who has the better Viennese Table?
It is especially NICE to see the Bas Mitzvah girls take on a chessed project to make the occasion really special.