This is a bad idea for a few reasons...
If MS built Win RT with emulation to run traditional keyboard/mouse optimized applications they would basically be creating a crappy, inefficient, and redundant version of Windows 8 that runs on very weak processors (ARM).
Emulation and virtualization have a certain amount of overhead and performance penalties caused by all of the translations that have to occur between software levels. ARM processors are very weak compared to mobile x86 processors. It's one thing to have a more powerful system emulating a less powerful system, but the reverse does not work well at all.
Traditional x86 windows programs will expect all kinds of different APIs to be in place that do not exist in Windows RT. Things like the Windows Registry, print subsystem, graphical user interface APIs, a file system layout similar to normal windows, security libraries, advanced networking, memory management, and tons of others that collectively make up Windows 8. Every application out there depends on a wide variety of these APIs and without them the app won't run correctly or even start. If you add all or even the most common of these APIs into Win RT as an emulation layer you will end up with Win RT being more bloated than the full version of Windows.
Windows RT isn't just about running Windows on ARM processors. Currently there are a few different Intel Atom based tablets running Windows 8 that have battery life, size, and weight similar to an iPad. Also, Intel isn't too far away from releasing x86 processors that are significantly more efficient and deliver ultrabook performance. Windows RT is, in part, about having a very lightweight and inexpensive version of Windows that is strictly for touch only devices running on low performance hardware. It's designed to compete directly against the extremely simple devices that run iOS and Android for people that just want a tablet that is a tablet and nothing more. Trying to make Win RT act like full Windows defeats the whole purpose of what Win RT is targeted to do.
The full desktop in the cloud thing would be a big deal to maintain for MS and not cost effective at all. They sell someone what is supposed to be a cheap content consumption device and they have to maintain a full OS with all of the customers installed desktop apps in the cloud. Not cheap and not easy.
Having said all of that, I do agree that it would be good if MS marketed the ability to use Win RTs built in remote desktop capabilities to access the customers own Windows 7/8 computer. They could have a program/wizard that helps you set it up from both sides including opening the required firewall ports and setting up a route of some kind to the internet to allow external access to PCs behind a home router.