^^^Yup. When Edward joined the coven it would've been weird for him to have a different last name. If he and Carlisle had the same name they could play it off like family.
Rosalie was so attached to her human life and so stubborn to start with that she kept her full name, Rosalie Hale. This is also the point where they start becoming "foster children" as opposed to siblings or whatever. Clearly, Esme and Carlisle are too young to have teenage children.
Then we have Emmett. He and Rose fall in love. So it would be weird if they were pretending to be siblings. And illegal in some states. So Emmett would be a Cullen so no one would think there was any blood (no pun intended) between them. That would just arouse more suspicion than was strictly necessary.
Alice and Jasper then come in to the picture. Again, both in love - so the same last name thing would still be weird. Since Jasper and Rose look alike he decided he'd just be her twin and therefore a Hale, for the sake of ease. That relegated Alice to Cullen status.
I thought about them changing their names when they got married but that would just cause more confusion don't you think? It's hard enough for them to get one name, let alone change it. Not to mention they go to school as married couples. People wouldn't assume they're married, they'd assume they're siblings and REALLY disturbing.
I think when Bella joins them in high school, she'll become a Hale - so she can still be with Edward. She'd be Rosalie and Jasper's little sister. And Nessie, she kind of gets to pick whoever she wants, huh? She could be Smith, or anything she wanted. I guess she'd be a Cullen or a Hale though. But if they all went to school at the same time Carlisle and Esme would have... seven children. Hm... maybe Edward and Bella will stay home and play mom and dad too for that generation