What would she be gaining if she was faking it?
After watching the video, if Hillary was faking, she is one damn fine actress. She was obviously in distress, taking several minutes to gain her composure. The timing for this is very bad, right after she went on Jimmy Kimmel's show and laughed off the rumors about her health by opening a pickle jar. If this happens during the debates, there's no way the media can continue to gloss over it.
I supposed if she is asked a question at the debate that she cannot or doesn’t want to answer, she could use a history of coughing to try to avoid it. However, I doubt this would work out well.