I was recently in a pre-social dance class when I noticed some behavior that troubled me: a follower came in ten minutes before the end of class and jumped right in. Each leader she danced with had to contend with the fact that she had missed fifty minutes of instruction during which the instructor had laid the technical foundation underlying the connection and footwork necessary to perform the movement being demonstrated.
Would this be a good test of the leader's ability to socially lead the figure? Of course.
Was it fair to the other students, including the already unbalanced number of followers who had to practice on their own as another leader was taken? No.
Was it fair to the instructor, who had to decide whether or not to devote energy towards trying to catch up a student who was so far behind the rest of the class? No.
Showing up on time for class isn't just the best use of your time and money- it shows the other students as well as the instructor that you respect their time as well.
Perhaps it's my own background in other dance forms that makes me particularly sensitive to this. In the ballet world, if you enter class late you are expected to wait on the side of the studio until invited to find a place at the barre so as not to disrupt the exercise currently being performed. In some cases, if a student is too late, they are not allowed to take a place at all because they have missed a sufficient amount of warm up that they are not mentally or physically prepared to participate in class.
So remember, when you're in class and working on your own dancing- everyone else in the room is working, too.