a publication from Ner Le’Elef (*) touches on the idea of osek b’mitzvah, and does not find it fully convincing as a halachic instrument. I frankly did not mean it as a strict halachic argument, just a sentiment that they seem to find at least reasonable.
WOMEN’S ISSUES BOOK TWO Women in Mitzvos, p. 32-34
In a sense, a woman is considered to be continuously עוסק במצוה , and we have a principle that Osek BeMitzvah Patur Min HaMitzvah. Indeed, there are some Rishonim who hold that even if one has a lost object in his house it is considered sufficient to exempt him from doing another Mitzvah like giving Tzedaka (even though he could actually manage to do both mitzvos simultaneously) However, most Rishonim disagree with this approach. They hold that the definition of an עוסק במצוה is quite stringent. As the Meiri (. ברכות יא ) …. “one who is engaged in one Mitzvah is exempt from a second Mitzvah, [they only intended this to apply] so long as he is still actually involved in the first Mitzvah, and the second Mitzvah can only be done at the expense of the first.” … We would then question why a single woman or a widow whose children are all grown up etc. would have these exemptions at all. Perhaps these Rishonim would say that a woman is exempt from time-based מצוות because her entire approach to serving G-d is one of constant readiness to respond to changing realities. For example, although one can easily provide for children’s physical needs according to certain schedules, if one is really going to allow children to develop as unique personalities, fulfil their creative faculties and fine-tune their moral sensitivities and values, it requires constant alertness, availability and engagement. Under such circumstances, even when one is not actually busy with the Mitzvah one would be
exempt [Sukkah 25]. In addition, one may ask why women should only be exempt from Zeman Gramas. According to this approach, a woman is considered permanantly Osek BeMitzvah and should therefore be exempt from all other positive mitzvos as well
This is answered by saying that it is true that a woman is not always considered Osek BeMitzvah. However, her situation is such that she can never know when she is going to be Osek BeMitzvah. Therefore, it was specially time-based Mitzvos, which require a woman to free her schedule at a particular time, from which she was exempt. And even though many time-based mitzvos can be done for a whole day, a woman can easily be busy the whole day. The Torah did not want her to have to figure out just when she is Osek BeMitzvah and when not since even Tirda Demitzva is enough to exempt one ( סוכה כה ). However, this is difficult. For we do know that the Torah, for whatever reasons, did obligate a woman in some time-based Mitzvos. Now, although a woman does not have a general exemption form these mitzvos, why can she not exempt herself from them whenever she is busy with her family? And the same goes for any other mitzva with which a women is obligated. Perhaps we can modify our original logic by saying that, although a women isn’t truly Osek BeMitzva when she takes care of her family, but the Torah wanted her to get the message of how important this is, and regarded her, with respect to time-based mitvahs, as if she is an osek bemitzvah. This might also explain why all women are exempt from this category, because it isn’t because of the actual business with her family that causes the exemption, it is the message of how important taking care of her family is, which is the source of the exemption.