It may be that they just ran out of time and hence had to leave out full desktop support for RT. I think that support will come but first they need to fix the performance issues on RT. It looks like a lot of third-party and MS developed apps have frame drops and appear choppy. Most of these apps are HTML/JS written apps so may be it's limited to that.

On the Start screen if you scroll through the tiles very slowly (I am doing this deliberately) you will notice that the tiles jump by 3-5 pixels instead of a smooth scroll. Start Pinball FX 2 and then switch to the start screen. The frame rates are really bad. This is one thing that Apple does well. They don't release products like that. I am sure the devs at MS already know of these issues and hopefully will release a fix soon.