I managed to keep Kate alive for a full 6 or 7 days before a massive zombie horde spawned around my house without me knowing and I opened the door right into them.
But ya, after the initial shock factor of smothering her (like, why can't I just give her the pillow to get comfortable?) it saves a lot of time, albeit you still can't do anything for the day until you go to bed, cause it's nearly nightime :/ If anything, if you really want Kate to be alive, only keep her alive to serve as a distraction for the raider that comes into your house. Then bust his brains out and take his bat and shotgun.