I've been reviewing my trusty guide Self-Editing for Fiction Writers by Renni Browne and Dave King. *If you're a writer and you don't have a copy, go. Get. One. Now. You won't regret it.* I'm a multi-tasker (aren't all moms?) so I was reading on Dialogue while looking up Proper Punctuation Usage in the web. My internal editor has been kicking in lately and I just have to get these things out of the way before I resume writing.

Last year, a very competent  writer friend (I respect her command of grammar) pointed out to me my use of comma splices in dialogue.

*Comma Splice: the use of a comma to join two independent clauses where the clauses are not connected by a conjunction, semicolon, or period. Example:

              I have a monkey, he likes to play with me.

By the rules of proper punctuation, this should be:

             -with a semicolon:   I have a monkey; he likes to play with me.
             -with a period:         I have a monkey. He likes to play with me.
             -with a conjunction: I have a monkey and he likes to play with me.

(I really don't have a monkey, in case you're wondering, but my Grandpa did when he was alive. Needless to say, he was a super cool grandpa.)

Okay, now that we've established that, let's look at what Self-Editing has to say about making Dialogue less formal and less stilted (pp.101-102):

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                Another helpful technique is to use sentence fragments. Consider this exchange:

                "Is she pregnant?"
                "It doesn't matter whether she is or not. She's not going to marry him."

                 It sounds much less formal--and more like real speech--if you edit it to read:

                "Is she pregnant?"
                "Doesn't matter whether she is or not, she's not going to marry him."

                In the second exchange, the writer has used another technique (in addition to the sentence fragment) to good effect: the two sentences in the answer to the question are strung together with a comma instead of the (grammatically correct) period. If not overused, this technique captures the rhythms of real speech remarkably well.
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I think the key here is in the last sentence of above passage: If not OVERUSED. Grammarians will still balk at seeing comma splices in dialogues, but for us lesser beings, this is good enough proof that it can be acceptable. As long as we don't overuse it.

As with all things in life, aim for balance. Listen to your gut. Keep yourself true to the voice of your story. 

Happy writing!

P.S. If you have any questions, enlightenments, disagreements, and rantings about above topics, please feel free to comment below. I'll try to answer as best as I can (armed with Google, Strunk and White, and Self-Editing for Fiction Writers).