Yep. Some of us can.
I make a distinction between informal communication (such as conversation with friends or Usenet posts) and formal communication (such as business letters and essays). Fiction can use either, depending on context.
I would certainly not want the impersonal third-person narrator to "speak" colloquially, but it lends a pleasing verisimiltude when characters do so, especially if they are from the "lower orders" of society, as is so frequently found in police procedurals and "hard boiled detective" novels.
It can be said for prose, as for people, "It takes all kinds."
Cindy Hamilton