I heard a story from an electrician who was finishing a job started by a second electrician. The stucco crew covered an outside receptacle and the customer decided that he didn't want the receptacle, anyway. The first electrician phoned the second electrician for information. The conversation went something like this:
1st: Where is the feed for the plug? 2nd: It's at the panel. I marked all the wires. 1st: Yes, they're all marked, but it isn't there. 2nd: Maybe I fed it from an inside plug. 1st: Which one? 2nd: Try the centre of the East wall. 1st: Okay, got it. By the way, It's code that you can't feed an outside plug from an inside circuit. 2nd: I know. I always use a separate circuit for outside plugs.
Is it possible that electricians who claim perfection have really short memories?
The short answer about the outside receptacle is "Christmas lights". I gave my wife a dedicated 20 and she managed to pop it. Some of these heavy duty electric lawn tools pull close to 13a and even peak higher under load.
twh, you've hit the nail on the head: it all comes down to the likely use - and that, alas, is more of a design issue than a code issue.
I think back to the home where I grew up. In that home, the outdoor receptacles (remember the ones at the base of the post light at the end of the drive?) were only rarely used, perhaps for the hedge trimmer, or some holiday lights. Maybe a trouble light as you worked on the car.
Now, fast-forward to the house I wired a few years ago. The patio receptacle has been used as the supply to a pond / waterfall, as well as an 'BBQ center" complete with refrigerator. Holiday lighting has expanded to more lights than a 60's airport needed. Nearly everyone uses an outdoor receptacle as the source of power when they build a shed - and only heaven knows what's inside those sheds. Suddenly, the idea of a dedicated circuit for the outdoor receptacles isn't so silly after all.
I have an electric smoker that pulls 1650W ... with a 5-15 plug on it. That is where another 20a circuit went. It is an outdoor kitchen so I should probably have 2. There is another 20 out there but it is shared with the bathroom light,fan,heat lamp circuit.
I have another, shared with my mini-split for the HVAC service outlet. The boat lift circuit also picks up the 2 on the south end of the screen cage and another circuit picks up the north end of the screen cage and the shed. The shop circuit picks up the ones in and outside the pump house. The design goal was to never be more than 50' from water and power, anywhere in the yard and have enough current available to do what I need to do. There might still be a spot but I am not sure where it would be.