Microsoft said this to The Verge recently:
The scenario outlined is not a security vulnerability and does not pose a threat to Windows RT users. The mechanism described is not something the average user could, or reasonably would, leverage, as it requires local access to a system, local administration rights and a debugger in order to work. In addition, the Windows Store is the only supported method for customers to install applications for Windows RT. There are mechanisms in place to scan for security threats and help ensure apps from the Store are legitimate and can be acquired and used with confidence.
We applaud the ingenuity of the folks who worked this out and the hard work they did to document it. We’ll not guarantee these approaches will be there in future releases.
Only do this if you want to run unsigned apps!