Are Professors Supposed to Attend Class?

After the last three days of attending school at UB I have developed several answers to the question stated in the title.
1. Only if they remember
2. Only if they feel like teaching on that day.
3. Only if there is power
4. Only if there isn't power
5. Only if they do not have anything better to do
6. Only if it is for 5 minute
7. Only if it is to tell students that they are not allowed to have cell phones and that if you care about learning their name they have to go find their office and look on the door
8. Only if it is not a 400 level class
9. Only if its cold outside
10. Only if its hot outside
11. Only if it is not for exchange students
12. Only if you make the students wait for 45 min in a 50 min class