By being a porch it implies it contains nothing more valuable than some muddy boots and a wet jacket.

According to who?

On what basis do you make that assertion? I do keep shoes in there, yes. I also keep mail that I haven't yet picked up and sorted, I also keep keys in there, on a rack (out of sight and reach of the letterbox. My wife also has a few pictures of the family and a nice vase on the windowsill.

Who are you to dictate to anyone what their porch should be used for?

If you have your dog in there, I'd strongly suggest you lock it.

Why? She knows not to open it herself. We trained her not too.

Am surprised this has never come up in real life though.

It has, once. Delivery driver opened the porch to drop off a package. Dog ran out and jumped on him (in a friendly way).

I recalled the dog, told him that he shouldn't have opened the door, asked if he could ring the doorbell in future.

He apologised and went about his day. No one got angry, no one got upset. And he definitely didn't argue back.