In general, I think that the better professors are the ones with greater class discipline in that regard. I was a teacher for a few years before law school and in my opinion, classroom discipline is as much of a problem in law school as it was when I taught seven and eigth graders.
I generally don't care whether students return to class after a break. I do care, however, when students come to class late by walking through the front doors, or when students leave and then return through the front doors. Besides being disrespectful to the professor, it is a huge distraction. And it only happens in classes taught by professors who let it happen. I have generally enjoyed and have better paid attention to professors who prohibit students from entering the front doors of the classroom after class starts and who prohibit students from entering and leaving. Emergencies exist, but somehow they happen with greater frequency in classes taught by teachers with little to no discipline.
I don't think it works that way in law school. I've been a teacher as well and I think at the law school level it's more about gaining the respect from your students than treating them like they are not adults. None of my profs had an attendance policy last year and because they were all well-liked, engaging, and exerted enough authority that students came on time, didn't leave class in the middle, and rarely skipped even when the prof was teaching material that wasn't going to be tested. The professors who have a more juvenile style of discipline tend to need it more because the classes aren't engaging enough to get people to come in voluntarily or on time. However, discipline at my school is nowhere near the problem it is in middle school.