I'm over my head, here, because I don't have inside insight, but I wouldn't be surprised if Microsoft weren't to provide an update that allows more flexibility in designing the Start options, at least for the Pro and Enterprize versions. As for the different versions, there are significant differences in functionality, so users don't have to pay for features they don't want. But technically speaking, they are all the same core O/S with common user interface.
Windows RT is limited due to running on the limited ARM processor architecture. There are always solutions for the tech-savvy, but I think that most "Enterprize" users find the current tablet architecture limiting. It has great portability, and makes a nice adjunct to the desktop system - maybe even used more than (or instead of) the desktop, depending on the individuals role in the organization.
I like tablets (this is written on an iPad). But there have been numerous times when I lamented the fact that I can't do this or that on them. The "what if" factor of thinking about what it would be like if I could do everything I can on my desktop, with my tablet, intrigues me a lot. I believe this is where Microsoft is going. The technology is not there yet - because Intel has not yet released its new generation of mobile processor. As a result, the first generation of Surface Pro will not have the battery life of its ARM competition. But it will be capable of doing a lot more. And eventually, soon...
Imagine a tablet with the full power of your desktop. Now imagine your office, with that tablet connected to a large Hi-Def screen, and a wireless keyboard. The tablet is placed flat on your desk (a much more comfortable reading position, by the way), just above the keyboard, with the large screen monitor (perhaps also touch-capable) just beyond that. The tablet's touch-screen becomes your mouse. You are typing, and now you want to insert text somewhere else. Instead of having to lift one hand from the keyboard, place it on your mouse, locate the mouse-pointer on the screen, move it to your insertion point (possibly having to also use the scroll wheel to find it) and click it - you simply touch the tablet screen where you want to insert your text, and voila - you are there!
Then when you need to travel - you just take your tablet with you... How cool is that?
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