Here is why, and why I personally believe this product might be great.
I will never carry an iPad when I am out and about. If I am going to carry a device of that size, I will bring a laptop. The only use that I personally have for an iPad is laying in my couch or using it as a stationary device for a baby call or something like this.
In this scenario, the bigger screen size is nice.
I often find myself on the go, wanting a larger size screen to deal with random incoming stuff, accountant, customer issues, PRs, and so on. In those cases, it is very handy to have a larger screen. I can manage with my current screen size, but it would definitely be more productive with a larger screen.
I am personally worried about the reliability of the mechanical parts, and I would be really annoyed if the screen started to crease, as I have seen similar devices do before. But if Apple somehow magically fixes that problem, then I would be inclined to buy a product like this for my next phone purchase.