I listened to the latest Vector podcast, and I think they missed one big thing (the biggest, actually) driving Asian demand for phablets.
Our family went to Hong Kong for vacation a year and change ago, to visit family over there. And the biggest thing that struck me was how many people were watching TV shows on their phones on the MTR (Hong Kong's metro; they were going to and from work, presumably). Seriously, it was a large majority of the young people.
And that's a big application that will look best in 1080p (I'd hate to think how big the tablet would need to get for 4K to make any sense, even ignoring the streaming bandwidth demands of that). If you're going to spend half an hour or an hour every day watching TV on your phone, you'll damned-sure want to do it with maximum resolution. So I can certainly see why people would want a bigger phone.
(And while I'm talking about this, let's also point out what this says about our crummy public transportation, that we can't even offer free WiFi on trains and buses. And what it says about our telecomm infrastructure, even more than what it says about our transportation infrastructure.)
The downside, of course, is that the phone looks ridiculous when you're using it as a phone. I remember one young lady using one of those phones with a case on it. It looked like she was holding her purse up to her ear to talk. Looking cool, she was not.
All that said, if Apple were to make one, would I want it? I'm leaning towards no, but I'd definitely have to hold it and play with it a bit to be sure.