A handful of folks out there really wanted to play Mirror's Edge. They wanted to love it as much as the franchise's biggest fans do. However, they were hampered by terrible motion sickness, rendering them too queasy to run rooftops for more than a few seconds. There's only one option available to those who get sick easily: third-person mode.
Now, would a third-person perspective change the entire experience? Absolutely. However, it seems unfair to leave out those who can't handle the fast-paced first-person action without getting sick, so throwing in a third-person option would be a nice gesture. In theory it'd probably play much like Assassin's Creed or Uncharted, and the new perspective wouldn't be mandatory. Still, even if you weren't prone to motion sickness, the mode would make a second playthrough feel pretty fresh.