The concept of at-will employment always favor the employer and never favors the employee, because in short, at-will employment, means that either party, that is the employer or employee can sever their work relationship at any time, without notice, and for basically any reason that is not in conflict with a higher law. This means that the employer can at any time fire or dismiss you from the job that provides all of your income and/or health benefits without your consent, whereas you, as the employee, have the basic right to quit at any time, hardly a right that is equal in concept or in effect.
There can be exceptions to the full effect of at-will employment, for instance at-will employment can be superseded by State or Federal law, and in addition at-will employment is subject to certain exemptions, depending upon the State or the city of your employment, to wit exemptions such as Public Policy, Implied Contract, and Good Faith may apply. Also, there are certain job segments to which typically employees are not treated at-will which are government employees of either State or of the Federal government, and union employees that have enacted a collective bargaining agreement with the employer, which has specific rules attached to it in regards to the termination of employment. In the absence, of working for the government, or of being part of a recognized Union with a contract with the employer, most other American employees are employed at-will, whether or not they have signed a document to that effect.
While it is fair to say that most Americans would not disagree that an employer has the right to fire you for just cause, very few Americans would agree that your employer should be able to terminate you, without a good and valid reason, yet, at-will employment essentially means that your employment is in danger of being terminated at any time, for virtually any reason, or in jurisdictions to which there aren't further lawful protections, for no reason at all. That type of mindset is inherently unfair, and while non-government employment companies should have the flexibility to promote or to layoff employees at their discretion, that right should be tempered by laws that fairly protect the employee.
Additionally, in an era of merger and acquisitions, it is of enormous benefit to companies that are either looking to acquire or looking to be acquired to have their labor force setup in such a way that their employees are all classified as at-will, because that gives the acquirer of such a company the powerful flexibility to terminate, layoff, or let go, any and all employees not deemed redundant or necessary for their continuing operations of the merged organization, which often times means a massive savings in terms of labor cost.
In today's world, many employees survive literally paycheck to paycheck, to which if they were to lose their employment; this would have an immediate and detrimental effect upon their lifestyle and ability to take care of obligations. Most employees only want from their employer, a fair deal, nothing more and nothing less, so that if they as an employee have done their part, they should then expect in return that their employer should honor theirs as well. A handshake between a new employee and employer should mean something, and what that should mean should supersede the bogus conception of "at-will".