Perhaps as a point of sale device in shops?
Finally... someone mentioned it.
This isn't a "Pro" device in the "production" sense.
It is a "Pro" device in the "profession" sense.
Displaying larger documents or graphics, plans, charts, etc at a reasonable size, instead of paper, with the option of mirroring to an enabled projector.
Kiosk duty.
Point of sale duty.
Other specific-role duties.
Smaller iPads are personal multi-use devices, that many are adapting to professional, educational, or other specific-role uses, some replacing old-tech like cash registers or multi-component kiosk computers, some are completely new roles that computers didn't do before, and move towards paperless uses.
Whether a 12-13" iPad would be for portable use, or affixed in a specific location, it isn't likely as targeted at personal hand-held longer-term uses as the 9" iPad air, or the more mobile iPad Mini Retina.
A 13" screen is also an easier display to not get as close to, or showing to other people whose eyes aren't within 36" of the screen.
Don't think of it as a bigger, heavier iPad Air alternative for your personal use... think of it as an alternative to iPad Air for other use roles and collaboration, rather than personal use.
Whether laying flat on a surface, in a standing case, or on a device mount... weight might not be that much of an issue, in terms of holding it for long periods while using it... and even if it is 2 lbs., that is lighter than most pro laptops to carry in a bag from point to point, just for presentations and collaboration. Even with a bluetooth apple keyboard, it still may come in at an acceptable carry-weight.
And for some of the other criticisms in terms of iOS usability... there is nothing to suggest that an iOS update won't have usability upgrades and new features to go with the expanding real-world implications of a device like this. Just because iOS has been a certain way, doesn't mean that it won't expand, as processing power on these new devices exceeds previous generations.
Condemning a new hardware paradigm variation due to current and past software limitations is short-sighted.