It makes a ton of sense if the iPad is cheaper than the Surface. It stops making sense because of Apple's pricing model.
The original argument for tablet computers in the first place was that sometimes you don't need a full-fledged computer, so you pocket $400-500 and get something cheap instead that's a limited but still quality device. And I think that would still be true with the iPad Pro. You want something that's closer to a full computer, with a real keyboard, but you still want to pocket some money.
Except... the pricing is awful.
It's like they designed a product for a real niche, and then stopped thinking about that niche as soon as they got out of the design phase and started working on implementation.
As a point of comparison, even if you hate Windows with a passion, you can get the new Macbook Air they just announced for basically the same exact cost as an iPad Pro + keyboard, and that comes with a full-featured OS. Is a Macbook Air as portable? Maybe not, but it's still pretty darn portable.
You're giving up the pencil, but you're getting a better OS on a computer with better specs. But I guess if you're a digital artist, and you prefer the iPad pencil over the Surface pen or Wacom tablets, and having a full OS doesn't matter for you -- this might be exciting. That seems like it would be a narrow demographic, but what do I know? I'm sure it'll sell well.