Not knowing could have been a very big deal. Say two people are in bed and one of them dies in his/her sleep....
Then you don’t go into bed with anyone else, at least to sleep (once again, I apologize to the ladies — this includes Daryl).
Simple precautions could easily prevent 99% of the problems stemming from sudden death. Have people sleep in locked rooms — non-zombies can let themselves out, but the newly minted zombies would be stuck. Don’t go into a locked room until the person on the other side says their name. Obviously there would be situations arising from things like car accidents, but, overall, with reasonable precautions, common communicable diseases pose a far greater threat than the threat of sudden deaths leading to new outbreaks of zombieism.
What bothers me about this is that it is so unnecessary. It doesn’t advance the plot, it doesn’t add to the character development. There is no shortage of ways to realistically provide danger. And this type of poorly thought out writing runs throughout the series, and detracts from an otherwise compelling story.
What would be really interesting, however, would be this: the disease is airborne, but can, at any time, cause death. Here you have a situation where at any moment, the person sitting next to you can turn. What is more, every person knows that they are fatally infected. Now that would provide a more realistic cause for concern.