They're probably working on such a device to be prepared if larger format tablets from their competitors become all of the rage. After all, the market for tablets is rapidly evolving. Think of it this way: the first tablet with mass market appeal, the iPad, was released about 3.5 years ago and even then it had a slow start. If Apple took 6 months to respond to a competitors product, you're talking about being behind the trends by about 25% of the history of modern tablets.

Will large format tablets become popular? I think that they may. It is entirely possible that iOS and Android will replace OS X and Windows for most consumers. It is unlikely that they will do so in the office users and for demanding home users, but even those users will likely use iOS and Android devices in addition to traditional computers. If that is the case, it helps to have a broad enough range of products to fill the various market niches.