interesting, my instinct would also be to have a sliding door on the outside of the coop but with what you have said here on the inside makes far more sense with an automatic door opener. Since I am debating getting one that is good to know.
I also have mine on the inside, though in my case I was more concerned about the possibility of the door freezing in the rails, and just the general wear and tear of the elements outside vs having the door (mine has wood rails) inside.