(California has a carve-out in state law: an employer can't claim stuff that was not done on company time, nor with company resources, and which does not relate to the company's business. But many states, perhaps most, have no such carve-out. And even the California rules are subject to interpretation; what isn't related to the company's business if the company is Google? So, it's wise to consult a lawyer regardless...)
Trying to fight your way out from under a noncompete, or lawyer up on account of a side consulting gig or independently conceived and developed idea, really stinks. And the further you get in your career the more likely you'll encounter such a situation.
Such cases ultimately seriously dampen both enthusiasm for the employer, and the odds of finding talent interested in working under such a regime.
Of course, the ultimate negative would be for California's exceptions to be attacked by corporate interests.
Georgia recently amended our constitution to make non-competes much more likely.
I and much of the tech scene hugely lobbied against this effort, oh well. We didn't need to work for in state companies anyhow.
Those sorts of clauses really suck the fun out of life. "Gee, I'd have a hobby and share my work with the world, but my employer owns everything I do. I think I'll play video games instead. They can't take that from me!"