I could ask them, "and why did you not plan for this earlier (like every other student in your major)?" Or, "And the reason I should make an exception for you is?" But doing this only makes the begging, whiney conversation last longer. They aren't there to listen to reason. They are there to beg because they failed to plan.
If there truly is no room in the class (lack of computers) I will say no. But most of the time, I sign them in. it's as if I enjoy the self inflicted headache.
It's nearly guaranteed that this student will come to class late (if they come at all), not listen/complete assignments accurately, complain about something or another about my teaching (blame me for their inability to listen) and then give me poor marks on the teacher eval.
Why do I let them in?
Is it because I believe their plea that they will work EXTRA hard?
Is it because I feel the need to help them graduate because they failed to plan during the 4 - 5- 6+ years they were in school? NO
Is it because I enjoy the extra day of my life (on average/student) I spend on grading their assignments/papers/projects? ABSOLUTELY NOT
I think it's because I am constantly running mini-social experiments in my head. And I love to have examples to give the regular (meet-deadlines) students about how they ARE better prepared for the real-world than others (students at hand).
A few years ago I had a sneaky McSneakster "trick" his way into several courses that required pre-requisites that he did not have. He was a double major (marketing) so he was sneaking in for personal gain. Roland was his name. He was the only plead-maker that actually proved to be worth the signature and time spent listening to the groveling. He added to projects and class discussion. His addition to my courses was not a waste of the day or two of my life I spent grading his assignments.
Will there be another Roland? Maybe. But not this semester. This semester's pleader already came late and admitted he/she was hung over. Point in case. New location, same sad story.