DD1.... her dad and I weren't married, and a big reason was that we couldnt' agree on a shared last name. When I was pregnant, we decided that if it was a girl, the name would be "First name, Middle Name, Dad's last name as middle name, Mom's last name" with the usage being "First name, Mom's last name."
When I married DH, we were both professionally established with our own last names and both kept them, and because I already had a daughter with my last name, and he was the last of his line, we decided that whatever gender, the child would have his last name. There was a strong possibility that it would be his only biological child.
Our third child will have his last name as well. One of the reasons that I don't push for my last name to be in there is that our last names are actually quite similar... his is only four letters long, and the first three letters of his last name are the same as the first three letters of my last name. And we know this child will be a boy, so the reasoning for DD1's naming isn't relevant.
By normal naming standards, my dad and his brother would be the "last of their line"... but my sister's husband took our last name, and their daughter has our last name, so my dad actually has two girl grandchildren with his same last name. (And on my mother's side, both her father's and mother's last names are all in no danger of dying out...she had seven siblings...) So I feel less urge to "continue" my dad's last name with more kids, especially when I know that passing on his name is important to my husband.
We've had occasional issues for DD1, mostly related to her grandparents on the "other" side, where they don't recognize her full name and drop my last name from hers. This was only really an issue when they created a gift trust for her, but was quickly sorted. I think they're probably a little miffed because both of their other kids are girls, one of whom took a married name and the other of whom isnt' likely to have kids, and K is the only child of their son, but that's his choice you know?