I don't see anything wrong with giving the baby the mother's last name. I've known some married couples that gave some of their kids one last name and some of them the other, for reasons of fairness. (The moms in those cases had kept their last names.) I think some girls name the baby after the baby dad because they hope it will make him stick around. Big hassle to undo it later if they guy takes a powder.
I'm giving mine my boyfriends last name. Because we are together and he is going yo be here to raise him/her I feel its the right thing yo do. If he wasn't around it would get mine