My two sisters and I all have middle names. In fact, Landen has two of them (I have a secret hope she marries someone with a hyphenated last name and adds it to her own so that her legal name consists of six names. Is that mean?).
My mom and her sister both don't have a middle name, but it was years before I found out that it was considered fairly standard not to give girls middle names (the horror!). My mom obviously made up for it with us, although I don't know if my aunt's three girls have middle names or not.
Bart's sister does not have a middle name, which I remember her mentioning early on really bothered her (especially since both of her brothers did (and when Bart's mom remarried, both of his stepdad's daughters had middle names)). His sister now has three daughters and all of them have a middle name.
I know that sometimes the rationale for not giving a girl a middle name is that eventually she'll get married and take her husband's name and then she can use her maiden name as her middle name, but I do not buy this story. Also, many last names do not really make for awesome middle names (my mom's maiden name is very long and hard to spell).
Both Merrick and I dropped our maiden names when we got married and kept our middle names (after all the effort my parents went to in picking names that go well with our first names, who are we to dump them so cavalierly?). Who knows what Landen will do when she marries Mr. Smirken-Tolens-Jones-Smith?
You may have guessed (correctly) that I am a firm believer in middle names and that I have not once, for one moment in my life, considered leaving my daughters with only a first and a last name. I am an equal opportunity namer - all my children will have a middle name.