Theaters with reserved seating tend to have more tiered seating than other theaters, meaning, there is more clearance to see over heads. So there's that... Many, maybe most, theaters don't have reserved seating, at least in all theaters, yet. (movie nerd, here) Many around here in ATL have it, but it's not ubiquitous yet. I hope it never is. It's nice to decide to see a movie on a whim every now and then.
One negative effect of assigned seating is that it doesn't force parties to sit next to each other for popular movies, so there are often empty seats when there really shouldn't be.