Turisu said:

Isn't that white/green R2 unit seen later being lowered into one of the X-wings before the assault on the Death Star?

I feel that removing him from Tattooine undermines his personal journey somewhat. The least George could have done is replace him with a dinosaur in that scene as well. In fact, replacing all the X-wing droids with dinosaurs would add so much more awesome to the Battle of Yavin!

Dinos during the Battle of Yavin, scary thought haha

Don't be surprised if you see some of those dinos on tattoine in episode VII though, if only to keep the fans of the prequels happy, I don't think we've seen the last of them.