I've worked in an at-will state my entire working life (26 years working), and never once been fired for "no reason." I have only been let go once, and I admitted right then and there that I deserved to be let go. My attitude stunk, and I knew it. I was 19 at the time, and it taught me a valuable lesson about doing your job and collecting your check, and not fussing about what you think you should or shouldn't be doing (as long as it is legal). If you are unhappy at work, you find another job; you don't go around with a bad attitude. Simple as that, lesson learned.
To me, "at will" employment works out to be similar to "no fault" divorce. Just because it is a no fault divorce state doesn't mean that people get divorced for absolutely no reason, it just means that they are not confined to a narrowly defined list of acceptable reasons. Same holds true for termination in at will employment. There usually IS a reason, it's just that they don't have to pick from a list of acceptable reasons and document them to protect themselves.