Echoing what xaenyth said – ‘As with any writing advice, it’ll work for some and not for others.’
I’m going to go way out on a limb and say Stephen King is both confident enough in his abilities and has a good enough grasp on what works and what doesn’t so that he can safely finish a ms without feedback. For an author that’s just starting out, they may need someone to point out problems early on, so they can fix them, rather than having to overhaul 100,000 words after the fact.
Personally, I can see the benefits to both. Sometimes it’s nice have someone look over the occasional scene as a litmus test — to make sure whatever’s in your head is translating onto the page. My WIP is my first attempt at true world-building, and having several people read through the first few chapters to make sure it’s not way too confusing because I haven’t explained enough, or way too overwhelming/boring because I’ve info-dumped everything into the beginning has been invaluable.
And…sometimes it’s just good for motivation. With my last ms, I wrote a crucial scene. A scene that had to be emotional and gut-wrenching and revealed the core of my MC. I had a beta read it before I’d even so much as looked at it after writing it. When I got back a ‘OMG! OMFG! Bunch of expletives….that was awesome!” type response…..yeah, that gave me more than enough motivation to stay up half-way through the night writing. 🙂
But (because there always is one)…good betas are hard to find, and I hesitate to waste them on a first draft. So many things can change in revisions, and your readers won’t get the benefit of reading through info that may not make it into a second draft. There’s something to be said for having a beta read your ms just as your ‘real’ readers would.