Sosa Sensei beleives that tardiness is a character aspect. Therefore, at my dojo, people who are late to class without a valid reason are severely frowned upon. I have noticed that the people who are consistently late to class also seem to be the people that have a hard time understanding the techniques, while the people who come thirty minutes early and spend their time warming up and helping new students settle in generally have a better understanding of what they are doing. This applies to everything from the lowest rank to 1st kyu, as I have never ever seen a blackbelt late for class. (this is my personal experience)
I can see how the rythm of the class can be broken by someone being late. Hearing the door opening and closing and seeing someone pausing at the edge of the mat with an excuse ready on their lips is just kind of sad. Most of the time, the excuses are something like "I tried o get here on time, but I forgot." Sheesh. Most of the time it is people who have been taking at this same time for several years.