When my colleague married his bride had a child from a previous relationship.
She could not change the kid’s name due to objections from the father.
She didn’t want to not take her husbands name.
He didn’t want to have a different name to his wife.
So they both hyphenated. I thought it was an innovative solution but apparently common in such circumstances.
Me neither. At the time I had to file my kid’s birth certificate, the father was still playing “It Wasn’t Me” and it would have been *illegal *for me to add his name to the birth certificate. US-MMV
Now that paternity has been proven, I could change the birth certificate, but I haven’t had the inclination to. As (I think) **Kevbo **was the first to point out, it simplifies form filling if the main caregiver has the same surname as the child.
US-MdoesV. Here you can legally give your kid any last name you want - yours, his, a made up name or Rockefeller-Kennedy. I’ve just not heard a social custom of blending for the kid when the parents aren’t blending themselves. Then again, we’re a pretty big country with a whole lot of social groups. What’s common in Philadelphia may be unheard of in Chicago. Or I might just be behind the times.
And, on the other topic, not only does it simplify paperwork and phone calls to have the same last name as your kid (Mrs. Smith is my ex’s mother, not me - I’ve never, ever been named Mrs. Smith, and I flinch when I hear it!) but also I never considered what would happen if I married someone else, took his name, and gave our child our last name. Now my son is the only odd guy out. I told him we could change his name, too, but for almost 10 years he’s said no. Still, it’s got to emphasize his own feelings of otherness, wouldn’t you think?
One girl I know has a surname like Torres y Lopez Schwartzman (no hyphens that I know of). So yeah, possibly she would too.
I thought about the “otherness” when I had my second child who has a different father than the first. Although I very much wanted the simplicity of having my 2nd daughter’s last name the same as mine, I worried a lot about the 1st’s feelings. So now we all have different names. It’s kinda funny that one kids’ teacher, classmates/friends’ parents & friends call me Mrs. R and the other group (in the same school & community) call me Mrs. B when my name is actually Mrs. K. 
It’s almost like having a secret identity. One day the school nurse called me about one kid’s paperwork and called me Mrs. R. Then five minutes later she called me about the other and called me Mrs. B. She sounded a little confused to be talking to the same person. (Yes I had my actual name on the forms next to my phone number.)
My mother married at 16 (due to my older brother). Naturally she took her husband’s name and gave it to her son (my brother was John Doe Jr). Mom’s first husband turned out to be an abusive drunk and eventually they got divorced. Neither mom or big brother wanted his surname. It was easy for her to go back to her maiden name. He was still in grade school and not only did my mother need to go to court but for some reason my grandfather did too. He ex wasn’t involved all.