It's certainly warming that Arcee accepts it as well. After her own choice to join the Autobots, Optimus had felt it necessary to include her just as seamlessly as he would to people who joined during the war. She was just as important, if not more.

Because she needed stability. He was already seeing changes for the better in her.

"That's right. Although I fear it's going to be difficult to maintain this now. I feel as long as we stick together as one, we'll have a more unified society here, and a more unified front to getting home."

...

He looks over, at Bumblebee chatting among the others.

"I'm fortunate to have him back."