Having the same OS for tablets and phones is not ideal IMHO. Apple took this route with iOS and so did Android, but from a development point of view it is problematic. The screen layout and relative size (i.e. the screen size in relation to the distance from your eyes) on a tablet is actually closer to the relative size of a laptop/desktop than it is to a smart phone. You hold a smart phone about the same distance from your eyes as a tablet, but the screen is much smaller and it's usually oriented in portrait layout. As a result, the UI for phone apps needs to be designed specifically for that layout. On the other hand, a touch app designed for a tablet will generally work just as well on a laptop or desktop monitor. A phone is also very restricted by the hardware/battery that can fit in it's form factor. A modern tablet can be nearly as powerful as a laptop.
I think that MS got this right. If MS had used a shared OS for both tablets and phones, the apps (at least for the UI layer, and possibly also to account for phone performance limitations) would still have to be written twice. A lot of code can be shared if the app is designed well, but there is no getting around the UI differences. Extending the desktop OS down to the tablet makes a lot more sense than extending the phone OS up to the tablet. In the MS ecosystem, I can write two touch apps (one for phone, another for Windows8/RT) and that will cover any device including phones, tablets, laptops, and desktops. Having tablets share the same OS as laptops and desktops also allows for innovative hybrid form factors that are not possible in the Apple or Google world. My Surface Pro is used as both a laptop and a tablet.
In the Apple ecosystem I would still have to write the same two apps, but they would only cover phone and tablet. To cover laptops and desktops I would have to write a third app that runs on OSX (which uses entirely different APIs). In the Google ecosystem the same applies except that there is no real laptop/desktop OS to write for. Apple is actually in an awkward position going forward in my opinion. They are going to have to decide if they want to extend OSX down to the iPad and make OSX capable of running touch apps (in other words copy Windows 8), or they are going to be stuck with tablets that are quite limited compared to what MS is offering. They will also have no capability to provide a hybrid device.
As far as the OP's question, I think Win RT exists for two reasons.
1) Tablet hardware is still developing and for truly small, lite, and cheap tablets with good runtime (i.e. direct iPad competitor) ARM is really the only option at the moment. Traditional Windows and the programs written for it won't run on ARM so something else was needed. However, Intel is very close to having x86 CPUs that will be a viable option for building tablets in that category.
2) This is just my personal opinion, but I think MS needed to be able to sell a super cheap version of Windows to tablet OEMs if they wanted to compete with iPads and Android tablets on the low end of the price scale $400 - $500. They could always license the full Windows 8 at a lower price for tablets, but what would stop OEMs from using that discounted licensing on other products. It would be hard to enforce, especially for the new hybrid devices like the Lenovo Yoga. For this reason, It's possible that Windows Lite (RT) could be around for a while and maybe even in an x86 version.