Last year I had decided to get either a tablet or an ereader. I decided on an ereader. I'm still a bit attracted to the idea of a tablet, so my first, unthinking reaction was, "Sounds great!" But then I thought about probable size. I had already decided that a 7-inch tablet would be too small for me, so I don't think this combination would work for me, even if it would for other people.
Then there is the weight question if they were permanently joined. If I just wanted to read a book, the ereader is all I need. The tablet part would just be unwanted weight. And I wouldn't want an ereader flapping around if I wanted to browse on a colour tablet.