Let's stipulate to the first sentence of your comment (and ignore everything that comes after it). If you're adding value to an organization, you shouldn't need a contract.
Wonderful! There you are, being a positive influence on your organization, knowing that you shouldn't need a contract. But! Suppose some manager doesn't recognize that you're being a positive influence? Or just wants to throw his weight around? Certainly, a person in a position of power who wants to throw his weight around shouldn't target someone who's a benefit to his organization. But perhaps he's perverse, or doesn't care, or views you as a threat, or (again) has made a mistake in his estimation of who's "adding value" and who isn't. You shouldn't need a contract in part because your superiors shouldn't do that kind of thing. But hey! They might do it anyway! The wonderful thing about being contract-free, or in an at-will employment state (i.e. almost any state) is that there's no such thing as wrongful termination and you have no recourse if you're fired---"but I was adding value!", in particular, is not going to be effective. In this sublunary world, you'd have to be delusional to maintain that if you're adding value, you don't need a contract.
Actually, I can't resist; let's look at the rest of the first paragraph in this light. How do you know that union employees in, say, Flint, were uninterested in "adding value" to the manufacturers who shut down the factories? People working manufacturing jobs---a skilled trade, you know---take pride in their work and I'm willing to assert, just as blithely as you were willing to assert the opposite, that they'd be plenty willing to try to improve the product, improve processes, etc., if they were but asked (and lots of improvements can, indeed, come from the rank-and-file in that way). It's true, of course, that when residents of Michigan are asked to add value to the company by accepting wages no greater than those commanded in Mexico, they are likely to balk.