Yes, but people say "ain't" and "irregardless" and use "was" when the subjunctive "were" would be proper, etc. Should we disregard good grammar simply because "people talk this way?"
In informal writing that's intended to portray things from the pov of someone who talks that way, YES! The secret, of course, lies in knowing when and how much for the piece you're writing.
In cases where the narrator is a character, narration is essentially dialogue; hence, a certain amount of leeway is acceptable. But I don't think I've ever read a story narrated by a non-character where bad grammar was allowed.
Read
Smokey the Cow Horse by Will James. That had an omni narrator who talked like the Old Prospector. I didn't especially like it, but it won a Newberry Medal. Okay, it was written in 1925. So maybe the Rules were different. But how about the First Law Trilogy by Joe Abercrombie? Plenty of colloquial narrative there, especially when we're in the pov of Dogman and Logen. The narrative associated with each of his pov characters followed different "rules." I thought it was pretty cool, actually. Got their personalities across.
Almost anything written in limited third with a zoomed-in narrative camera (whether or not we want to call it "deep pov," or something else) will include some informal grammar and usage in the narrative. The point is to write third person that reads almost like first.
Note: I believe a lot people forgive the misuse of "like" and "as if" simply because they don't know there's a difference. Indeed, someone above said they didn't even know it was an issue. As Roxxsmom said, "people talk this way," and as such, people don't recognize the error when they read it.
True, but like it or not, language shifts based on the way people actually speak. The linguists (and those of us whose moms were professional editors who constantly corrected their diction as children) gnash their teeth for a while and post angry lists about improper grammar usage on facebook, but the style manuals eventually get revised to reflect the new reality.
But I completely agree that breaking a rule of grammar intentionally, to create a character or narrative voice, is different from doing so haphazardly or because you don't know any better.