Speaking in Bad Grammar

I just finished reading a book that I got as a gift.  It was an interesting book; not my usual style but I did enjoy it.  The only minor issue I had with it was the occasional poor grammar, particularly in dialog.  It got me thinking about writing conversations.

The grammar issues were things that are fairly common in everyday speech.  For example, the first “as” in a simile was omitted in several conversations, as in, “His bathtub was big as a car.”  It doesn’t sound wrong, exactly, just a bit odd, even if it is technically incorrect.  Because it is heard in the everyday speech of some people, it is acceptable to include it in dialog.

But should an author write that way?  I understand writing really wrong grammar, or using excessive slang or clichés, to make a point about a character.  Does dropping in the occasional slightly incorrect sentence structure have the same effect?

I haven’t decided my exact opinion on this subject.  Do you have any thoughts?