From a security perspective (which is presumably the point when discussing locks), one would use different keys for all doors; or at least for all external doors. I understand the convenience of using a single key, but the author really should have cut a new key instead of using an existing one.
This sounds like common sense, but when you think about it, how is this more secure? You're going to store all the keys together anyway, so if you lose them, you lose them all. And it's not like a thief can make a fake key after picking your lock, they have to pick all your locks individually anyway... So how does it help if your front/side/back/garage doors all have different keys?
I grew up in a house with nine exterior doors (not very large, just every room had an exterior door) and getting them keyed the same was a wonderful upgrade.
Obviously a bad actor could get around it no problem, I’m just keeping out the honest