Sometimes when I'm rewriting, editing, or proofing my mind misses glaring mistakes because my mind automatically puts in the word or phrase I wish as I am reading it. Because I know what "Should" be there, my brain reads it as I intend, not as it would "really" read to someone who doesn't know the plotline, or that I mixed several different versions of a story into one, and mixed some of the details in an illogical manor. Having text to speech feature, even if it's not the most human sounding voice, can help me catch those mistakes. It would also help me get the feel for how the story flows, the rhythm of the words on the page. If I could assign different voices to different characters it would help isolate the dialogue and get the feel of that as well. Perhaps a character is speaking too formally to his wife in a private moment... or something of the like. Would really help the dialogue feel more natural.
Anyway, just an Idea.