I think this will be useful for thin laptops, tablets and small umpc type devices or small 6" - 8" tablets.

I think there is clearly a market for umpc type devices, devices like the gpd win prove it. The issue with such devices is battery life, graphics performance, availability of low power x86 chips, and cost, ARM processors can address all those factors. Honestly in my experience desktop interfaces are fine on small screens and Windows 10 makes it much better.

This can also benefit the desktop as desktop processor performance has been plateauing, AMD Zen will probably be the last big performance increase for x86 for a long time. Further improvement to x86 would require new manufacturing techniques and chip design not yet available such as transistor stacking and nano technology or new materials. Any improvements can only come on architecture level, x86 puts barriers to further improvement. ARM can improve desktop performance by improving performance per watt. Power consumption and heat limitations prevent substantial improvements in single threaded performance, ARM requires less cooling and has significantly greater performance per watt than x86. Desktops will eventually need to replace x86 and ARM could be that replacement.

Less power consumption can also mean for laptops smaller batteries with similar or greater battery life reducing costs. Reduced overall component costs enable more premium components such as higher resolution screens, better screen technology like oled or possibly screens that don't have wide aspect ratios. Other design innovations could be possible as well.

A dual screen tablet could me more possible as well thanks to lower components costs and improved battery life.

I am not hugely interested in convergence for phones as I prefer small tablets, but smartphones could run full windows while docked. Continuum could actually be useful for something.

I also hoping this has backwards compatibility with Windows CE, if not maybe it could be hacked in.

This could also be useful for portable gaming. There are several android devices with built in controls but they are super low end and the games are a mixed bag. Atom processors lag in graphics, where as ARM processors are constantly improving and could offer a better gaming experience. Windows has more advanced gaming frameworks so PC games could be ported to ARM and enabling full pc games on small device with decent performance, such games could be designed with physical input or pen input in mind rather than touch.