That analogy goes a bit too far. A smartphone is just a small computer with better connectivity. Outside calling, people use smartphones/tablets for very similar tasks: playing games/music/movies, sending messages, typing documents, giving presentations, etc.
Vise versa, people have started to use computers to call (Skype, Facetime, et al.).
There's a lot to say for creating one device that properly handles all tasks. I for sure know many people who would love to replace their phone/laptop/PC/mediacenter with one device and some docks.