I'm speaking from experience.

edit:

I was a TA for early CS classes. Sometimes we took attendance, sometimes we didn't. When we did, more people showed up, and more people passed. Teachers have a responsibility to teach both sides of the bell curve, and generally pressuring people to learn helps the lower side pass. I understand that it's kind of old fashioned and authoritarian from the perspective of a top student, but it really winds up better for the bottom students in general.