This doctrine wrongly assumes that the employer-employee relation is symmetrical. What is the reality? 

For a small percentage of workers, those with very special skills, the relations tends towards symmetry, and can even turn in favor of the employee: the employer can suffer a great deal if the employee quits. But in the majority of cases the employee is replaceable, and in the case where there is no shortage of workforce the employer has the advantage. 

In other words, most employees are in a lock-in situation with their employers. Their costs of switching jobs are too important, and losing a job can have disastrous consequences on their lives. Historically, unions have evolved to protect the individual employee from being abused by his employer, precisely because the relation is not symmetrical in most cases.

On the surface the at-will doctrine seams to be fair, but when we examine it just a little closer we discover the source of injustice: in the majority of cases the separation affects more the employee than the employer.

I am familiar with the neoliberal arguments against unions, labor market regulations, job contracts and others. Before you lose yourself in their rhetoric fix your mind on this principle: in a humane society well being and ethics comes before profits. Here is a little video that sheds more light on this subject.