In evaluating job offers for start-ups and for firms that have different forms of compensation (equity, stock options, bonuses, etc.) it is often a thrown around phrase that you should "get it in writing" when you negotiate something about the terms of employment.
For example, you may try to negotiate that you receive unrestricted stock instead of stock options; for a start-up that requires you to take a pay cut to join the firm, you may try to negotiate that after the next round of funding, your salary will raise to the level before leaving your old job; you may try to negotiate that stock options will vest more quickly than the current company policy; one very important point is to negotiate that equity expressed as a percentage of ownership of the firm will not be diluted by subsequent funding rounds and share issuance (known as "refreshing"); lastly, you may try to negotiate percentage of time that will be spent on different kinds of projects (a personal open source software project that the firm is interested in, attendance at conferences, etc.)
In all of these cases and many others, endless blogs, books, and career advice columns suggest that you "get it in writing" but I am not convinced that this would materially increase the chances of a company honoring such agreements when they are part of an at-will employment agreement instead of a binding contract.
For example, it could be put in your terms of employment that you get to attend a swanky conference in Hawaii every year. But then the company faces tough financial times and cannot pay for it. So you effectively just lose the dollar-valued equivalent of the experience, knowledge, relaxation, and travel from the conference. It would be like directly paying you less money. But under an at-will agreement, to my knowledge you can do nothing except for quitting or raising the concern with a manager or the Human Resources department -- usually resulting in no help in situations like that.
Yes, the company risks it that you might quit if they take away promised items from an agreement. But they might just wait until they can confirm you are in a life position where, for reasons ostensibly totally unrelated to your job agreement or the firm's situation (such as mortgages, children, tuition, etc.) the firm knows or believes you can't afford to take any action if they simply do not follow through on the agreement. And it would be very hard to prove that this was the firm's reasoning, as firms have nearly limitless plausible deniability when it comes to explaining why they are doing something.
Are there any legal conditions surrounding at-will employment contracts that would prevent this kind of behavior?
Is it valuable in any kind of tangible sense (i.e. legal enforcement, rights to claim wrongful termination or hostile working environment) to have negotiated such things specifically in writing specifically when it is a an at-will agreement?