The problem is that there is a very random factor involved here, and that factor is humans.
In an age of not taking responsibility for our actions, we are relying on people to 1) recognize they may be sick, and 2) take the proper actions once they suspect that they are sick.
I can imagine all sorts of scenarios, someone in denial about having it, someone that thinks they have a more benign illness and can't afford to take off from work, someone uneducated about Ebola, someone who denies the existence of Ebola, or someone who thinks that God will protect him from Ebola and that he will recover on his own.
Think of a somewhat possible worst case scenario: This person lives in New York city, takes 2 or 3 different trains/buses to get to work, maybe uses an elevator, visits a public cafeteria, doesn't wash their hands. How many people could be exposed in two days?
It might not be likely to happen, but if it does, how would you contain it?