Before I owned an iPad (or two as it stands now), I thought the idea of a larger iPad made sense. But now in using the iPad as I do, not just for browsing the web, but for actual productivity including quoting customers, working on costing spreadsheets, emails and the like, I don't think a bigger iPad makes sense.
The best and fastest input method is still the keyboard, be it attached like with a laptop or separate (BT), it's just the most productive way to interact with a computer. Sure, there are some things where touching the screen makes sense, but for the most part, the keyboard just eclipses screen only input for doing work (graphics work not included). So going to a "Pro" sized iPad almost forces the issue of needing a keyboard, turning it into a slightly lighter (maybe) laptop with removable keyboard.
There is also something entirely wrong about touch interaction with a screen at a parallel plane with your upper body. You use your hands and arms to hold the device AND you're supposed to be interacting with the screen - that's just not easy to do the bigger the screen gets.
And as for Microsoft's push for touch screen lap and desktop computers - it's a pitiful way of trying to incorporate technology from the tablet market into desktop computers.
I flip out any time someone touches my iMac or MacBook Air screen - because their grubby fingers leave smudges that then distracts from what I'm doing (ultimately making me need to clean the screen). I have no issues with smudges on the iPads or iPhone because it's easy to clean - but again, the bigger the screen, the bigger the issue it will be to keep it clean.