I really don't see why it should be a big deal that Rand Paul copied some lines from a (Public Domain) Wikipedia article in a speech.
I think it is much more damning that he admitted to intentionally sabotaging other medical students in order to help him do better relative to them in the school's competitive exams, and that he encouraged other students at his alma mater to use the same tactic.
Irrelevant of what I think about it, it seems Republican primary voters largely agree; Rand Paul hasn't been significantly hurt by this. I would argue his response is more disturbing/less presidential--the "just leave me alone" comment, for example, doesn't sound like a good crisis manager in action.