Surely a good compromise between "thumb-lock always accessible without key" and "need to find key in emergency" is "leave key in the door while at home and remove when going out"?
Granted there's an extra cost in getting an extra key cut, but means you get the convenience of always being able to unlock the door from inside in an emergency (provided you've left the spare in place), whilst also being more secure when you are out of the house as there is no key in the door?
The only minor inconvenience we have with this is that if the key is in the door then you can't unlock it from outside with the key, but since this only occurs when someone is already at home, you just get them to let you in