They are not obligated to don tefillin.
Please read carefully. I did not say they were obligated. No one argues that.
But the tosephoth on the gemara where the chachamim did not protest, they say a woman can wear them! (and that's quite obviously the pshat of the gemara or at least the most obvious one)... Hence the famous legend (not sure if it's true, but it's almost universally known) of Rashi's daughters wearing tefillin.
As I posted above there is no merit in doing something if it is not commanded of them.
Less merit does not mean "no merit." There is more merit to do something one is commanded to do, than there is for doing something one is not commanded to do. But that is a matter of degree. That doesn't mean doing something when not obligated has no merit? Who says such a thing?
Here is a discussion on the topic of Women studying Torah which is, in a way, analogous to wearing tefillin, etc.
I will post the beginning of the article below:
http://www.nishma.org/articles/update/updatesept92-wij.htmWomen and Judaism: The Question of Learning
Rabbi Benjamin Hecht, North York, OntarioOf all the issues that must be investigated within the realm of women and Judaism, the one that I believe is most significant concerns the question of learning. okuf sdbf vruw sunkw, the study of Torah is equal to all
(Mishna Peah 1:1), yet women are excluded from this command
(T.B. Kiddushin 29b). How we understand this exclusion must present important insight, not only into the role of women within Judaism, but also into the very nature of the mitzvah of talmud Torah.
The investigation could begin with the simple question: why are women not commanded to learn Torah? The issue is, however, further complicated because women are not just excluded from the command but, at least on the surface, seem to be instructed not to study Torah. See, as the primary source, Mishna Sotah 3:4, and, as a most definitive and powerful statement in this regard, Rambam, Mishneh Torah, Hilchot Talmud Torah 1:13. Unlike a mitzvah such as lulav and etrog or listening to the shofar, where women can choose to act if they so wish, the Halacha seems to decry a necessary difference in behaviour, demanding men to learn and women to refrain. Unlike a command such as pru u'rvu, to have children, where the distinction between men and women in the command is mostly of theoretical interest (although of great importance to the entire subject of women and Torah), the distinction in studying Torah is of ultimate practical import affecting, obviously, no less than our very approach to Torah education. This powerful distinction in behaviour in what is considered the most important of mitzvot is most perplexing. The question is not only why women are excluded from the mitzvah but how could there even be a distinction in this command? The concern is not only what this law says about the Torah view of women but also what this law says about limud haTorah.
The very value of Torah study would seem to be challenged. How can half of the Jewish nation be excluded from the pursuit of Torah knowledge? To say that they can be, ultimately, is to declare that the information to be gained is not really necessary for the maintenance of a Torah life-style. To say that they cannot be, however, contests the statements that declare that women should not learn. How do these two apparently conflicting positions merge?
The answer must lie in an examination of the limiting statements and a determination of what they really are declaring in regard to women and Torah study. For specific approaches and answers one may wish to see such works as Rabbi Elinson's HaIsha v'HaMitzvot, chapter 13 or Rabbi Moshe Meiselman's Jewish Women in Jewish Law, chapter 7. In general terms, the commentators discern two poles. On one hand, women must be able to learn and must learn those matters which she must know to maintain her practice as a Jew. The limiting statements never applied to these areas of study. On the other hand, they do recognize that there must be a restriction of some nature which must be defined. The issue, however, is the extent of one pole in connection to the other. It is here that the commentators disagree and the complexity of this law continues.
I will look into this issue further and may ask my Rabbi about it this Shabbat.