There's no difference between the ARM and x86 device except the processor though. Both are intended for the same thing -- a dual purpose device that you can carry around with you and use with touch input, and then sit down and put on your desk and use with a mouse and keyboard. The Desktop is still better for mouse and keyboard use for either processor.
The only difference atm is that the ARM version has less apps compiled for it. Personally, my guess is Microsoft plans to port the WinRT/Appx model to the Desktop too, so in the future you'll be able to install Desktop apps that can run on either processor.
That's just my guess, and depends entirely on what Microsoft means when they call the Desktop "legacy."