His point is, flat bars cause your elbows to pull away from you body a little, into car side mirrors. When gripping horns or drop bars, your elbows are naturally tucked in. Of course if your arms are straight on narrow flat bars, then the elbows aren't going anywhere. It all seems to be a personal preference thing.

The first paragraph is not a good indication of the OPs full thought.

I have flat bars because I'm cheap (came with the free bike) and not a messenger. I also have a front brake.