"It would be very helpful for missed calls. “Why did my wife call twice in the last 5 minutes, did something happen, should I panic?!” It also removes the need to leave rambling voicemail."

Well, what is stopping your wife from also texting you, if it was something important and she did not reach you? And what would force her, to use a potential subject line?

Otherwise it is an interesting idea, but I doubt it will be a killer feature, as most would simply ignore it.