@Anonymous It seems to be pretty unusual, but I've never had a guest who didn't have a clear profile photo of themselves, so I could easily tell that the person who arrived was the person who booked.
But if the profile photo was of something other than themselves, or it was a photo of a young woman, and a middle-aged guy showed up at the gate, I'd be asking for ID.
Never having had a third party booking, I do think I would deal with it on a case by case basis. Not all 3rd party bookings are nefarious, like booking for a friend who's been banned or for your 17 year old kid. I'd venture a guess most are made simply out of ignorance of the TOS.
So if the person who showed up wasn't the person who booked, I'd probably go on my intuition, and if they seemed shocked and confused that it wasn't okay and seemed nice and polite and decent, I likely wouldn't turn them away.
Like the sweet young couple who looked absolutely surprised and devastated when I pointed out she had booked for 1 and I only accept one guest at a time. It was her first Airbnb booking. I let him stay, they were content to squish into the single bed, and were lovely guests.