Windows RT will have a place, but its the same place that is filled by other tablets, smartphones and netbooks. It is going into a crowded market to offer web surfing, app games, email, web apps and light duty documents. Casual users will probably love it, especially if they have Windows 8.
The real crowd MS might have been aiming for, the business community, is going to avoid this product thus limiting its lifespan. The biggest draw of Windows 8 is the ability to use the metro apps between all devices, this is greatest weakness also. To get the full functionality of Windows 8 cross platform features requires all new hardware on the mobile side, at least. Companies have already shelled out for Iphones/iPads or Andriod devices and adjusted their IT infrastructure to work with these devices, then add that Enterprise customers not jumping on the WIndows 8 bandwagon right away, it really puts a crimp in the user base of Windows 8 RT.
Windows 8 RT and its assoicated hardware products will enjoy a moderate sales number. Its inter-connectivity could make it a major force, but its hit the market a day late and a dollar short to knock off iOS and Andriod based products.