Getting EC is more complicated than what most people think. You have to go to the doctor's office or hospital within 72 hours of the assault to be examined, evidence collected, and then obtain a prescription. Then you have to find a pharmacist that will actually fill the scrip for you, which as we've seen in other threads, is a toss-up. All this happens assuming you have medical insurance and drug coverage. Some insurance plans will not cover any type of contraception for any reason. Sure a pack of birth control pills is only $30 or so (I'm not sure about the cost of EC, but I'm assuming it's similar), but for someone in a lower socioeconomic bracket or a college student, $30 is a lot of money.
There are some side effects as well, none of which are pleasant. Nausea, vomiting, etc.
I found a website that has pretty comprehensive info on the subject and a good FAQ area.
http://ec.princeton.edu/