Some people are glossing over that "at will" is a double edged sword. Everyone talks about how the employer can fire you on the spot. The employee can also leave on the spot. In comparison. some countries require the employee to stay at the company for a period of additional time before they can quit. This could be months depending on how long they've been working.
Now does this employee benefit make "at will" worthwhile? Probably not.
At will employment is really the crux that erodes all other possibilities of strong worker rights. In most European nations, firing employees functions on a sort of whitelist principle. You may not fire your employee except in one of this specific set of situations. This also puts a burden of proof on the company to demonstrate cause for dismissal. The situation in (most of) the US is more like a blacklist: all reasons for firing an employee are valid except for this specific set of situations. Now the burden of proof is on the employee, to show his situation was part of the blacklist.
If any (or) no reason for dismissal is a valid reason, it takes the tooth out of any worker's rights law you might seek to enforce. If you cause trouble for the company you can simply be fired (for "no reason" of course). Yes, that's technically illegal, and you can sue and/or contact the department of labor. They now have to investigate and find proof that you were fired for an illegal reason. Whether you get justice now depends on whether the department of labor is adequately funded, how good (expensive) your lawyer is, how well the company covered their tracks...
This is why many people in the US complain that "they have labor laws, the main problem is lack of enforcement!" The structure of the system is such that good enforcement is required for workers to benefit, but businesses benefit from bad enforcement.
I don't really disagree with any of this, I'm just saying at-will is a bi-directional street, which I haven't really seen mentioned in this thread. Being able to quit at any time is technically a right that benefits the worker.
Now in practicality does this benefit most people? No.
You should know that when you want to leave, they will want you to leave too.
I mean if you're a nice person you'd train someone or make tech transfer, but that doesn't take months... So you being paid slackin around or you leave quite quickly?
The other double edged quality is that businesses may be more hesitant to hire anyone who is seen as risk if protections are too strong. Take France, where the youth unemployment rate is chronically around 18%. Some find work in the informally economy, where paradoxically they have even fewer protections.
I thought we were talking about legality, not physical restraint. For example, in Belgium an employee can be required to give notice of up to 13 weeks.