The reward is not having to work with that person. Probably not a good enough reward to justify the downsides, but still a reward.
I really wish there was a bit of a balance here in the US but I think the at-will stuff (along with stagnant wage growth) is why companies have such a hard time keeping people. Most of my working career has been (about 15 years discounting part-time work in high school) has been one in which I've worked with other people that very clearly knew or experienced that there was no such thing as stability in the modern workplace and you can be out on the street on a whim.
At least in the Duke professor's case, there was a process and a means of appeals it seems. There has to be a more human way for us to do this that balances the needs of people (and their faults) with that of a business.
But contracts not being renewed -- happens all the time in the "real" world. My point is just: good people don't get their contracts renewed all the time, often for silly or dumb reasons, and it's not some cosmic injustice because most people find another job.
The fact that finding one of these jobs is so difficult should not change the basic risk/reward stature of at-will employment.