SNARKY RESPONSE: But a smart man won't admit it out loud.
A brave man is the one that is not afraid to admit his mistake.
Mistakes. We hate to make 'em. We hate to have to correct 'em. But more importantly, we hate to be caught making one. Nothing will make a writer cringe more than hearing someone point out a mistake in your latest piece. Punctuation mistakes, grammar bumbles, plotting snafu's, and characterization gaffs can make a writer want to crawl under that rock the reader obviously thinks you climbed out from underneath. (Not that everyone who points out an error to a writer thinks the writer is a total idiot. Just that the writer usually feels like one.) The point is not to have to discover and own up to them after you put those pages out there for public consumption, or submit for publication.
Mistakes happen. No matter how hard we try, writers make mistakes while putting words to paper. Few and far between are the writers who produce grammatically correct and perfectly punctuated text the first time onto the page. In the clutches of the creative process getting the stalwart hero through that horrific confrontation with the dastardly villain far outweighs pausing to ponder silly things like punctuation and grammar.
And that's okay. For First Drafts. First Drafts are precisely that--the First of many before Publication. I won't pretend to tell you how many drafts are the optimum number of drafts, because every writer is different, but I'd suggest you need a minimum of four.
When you've got that First Draft complete and in the file, that's when you begin correcting the mistakes that slipped through the cracks with the clues left by your murderer.
Second Draft is for things like plotting, pacing, characterization. These are things that you'll smack your head over but have fun correcting. Well, sometimes you want to shoot yourself because you discover that the crazy aunt in Chapter 1 not only changed her name by Chapter 4, but also her sex by Chapter 7, and his residence by Chapter 9. Or that great fight scene you labored over for hours just doesn't work where it is in the story. In fact, and here's a real kick-in-the-gut, it may not fit anywhere, at all. And then there are the plot knots where you realize that amazing Chapter 10 couldn't have happened because you forgot to lay the groundwork for it in Chapter 3.
Drafts Three through Draft Number "If I have to read this again, I'm going to kill myself," are for further fine-tuning. Carving out chapters that don't work. Writing new ones that do. Killing off that character who's getting in the way. Combining a couple of minor characters into one multi-purpose side-kick. Dumping or re-writing that First Chapter because it no longer fits with the ending. Paring away about a billion extra words, because, less is more, more or less.
The Last Draft (whatever it's number) is the fine-tuning. When you've got the words right, you can then make sure they're all polished with perfect grammar and punctuation. I'm not saying you shouldn't be correcting grammar and punctuation as you go along. It's important to maintain the quality of your writing at all times. However, why labor over punctuation and grammar on sections you may end up heaving into File 13? No, fix 'em when you see 'em. However, you should dedicate the last and final draft to making sure that you did dot all those i's, cross all those t's, and punctuated every sentence perfectly.
And let me let you in on a little secret. Even after all that. Even after you've edited and proofed until you're cross-eyed, you need an outside set of eyes to read it over. You've gotten so close to your work that you probably won't see some of the errors, but that Beta Reader will, and bless their hearts, that's exactly why you have them.