It leaves some room for rather unscrupulous activities by some employers. I have known a lot of people who work in retail, and for whatever reason have had their hours cut to 0 or maybe 2 hours a week, to make them want to quit so that they won't get unemployment benefits. And employers can do things like change or extend working hours, increase workloads, cut benefits, etc, because "if you don't like it, you can leave."
Overall, I think "at-will" is a good thing. In countries with stricter labor laws, there are high unemployment rates because employers don't want to take the chance of hiring someone they won't be able to fire if things don't work out.
