At-will employment laws aren't an equivalent tradeoff -- which is why they are very uncommon outside the United States -- but the alleged benefit of such an arrangement to an employee is two-fold:

1. In places where termination is for cause, termination often causes an employee not to have access to various government benefits 2. At-will employment likely encourages more hiring in the first place, so it's possible that with more restricted firing you'd never have gotten the job.

I do not feel these benefits make up for the drawbacks and do not favour at-will as an organizing principle for industrial relations. But I just figured it would make sense to at least say the apparent argument.