Once upon a time, PDAs and mobile phones were different devices. Once upon a time, low-end point-and-shoot cameras and mobile phones were different devices. Once upon a time, music players and mobile phones were different devices. Once upon a time, GPS units and mobile phones were different devices. And to get a bit more esoteric, once upon a time barcode scanners and calculators and carpenters’ spirit levels and compasses and mobile phones were different devices. I only have so many pockets, and I have a lot of other things to carry too, so I had to decide which one or two of those I wanted to carry with me at any time (or at least, which one or two I wanted available in a second or two without having to get into my backpack); when I started carrying an MP3 player regularly, I stopped carrying and using a PDA. I still only have so many pockets and so many hands, so the pressure for convergence is still there. (In fact, there are tantalizing hints that smartphones and/or tablets may, with added accessories, start taking over the functions of desktop computers and/or laptops.)
There are still people who carry around a separate MP3 player or calculator or point-and-shoot digital camera or even PDA in addition to their phone, so I’m sure there will continue to be people who carry around a separate “prosumer” digital camera in addition to their phone for a good long time. (I’m one of them, at least for now; I’ve got a Samsung Galaxy Camera. But I still have only so many pockets, so it lives in my backpack, and I often find myself taking photos with my phone when I’d really rather have the zoom or low-light sensitivity of the Galaxy Camera.) But I absolutely understand the appeal of devices like the Galaxy Zoom, and if I had a DSLR that could also function as a phone, well, I might not want to hold it to my ear, but if I used a Bluetooth headset and voice dialling, and I were the kind of person who took more pictures than I made phone calls or sent text messages,I can imagine the temptation to use it as my phone and stop having to carry around the extra device.
I think the tipping point is when the general-purpose device (in this case the smartphone) does as good as or a better job than the special-purpose device (GPS, graphing calculator, barcode or document scanner, ebook reader, music player, camcorder, audio recorder, light-meter, heartrate monitor, prosumer or professional camera, wallet full of cash, keyring full of keys, flashlight, TV remote, etc.) that you’d otherwise choose to carry around with you in addition to the general-purpose device. The mix is going to be different for different people because they have different needs, but there are always the competing pressures of wanting a device that is really good at the particular things I care about, and wanting as few devices as possible.
And as far as tablets go, that’s the third Android device I have with me almost all the time. (Like the Galaxy Camera, it lives in my backpack, so it’s not instant-access the way my phone is.) And I find that while I have a lot of the same apps installed on both my phone and my tablet, there are some that I almost always use on my phone (because they’re only useful to me if I can get to them in a hurry, or if I can use them in a small space without inconveniencing people around me), and there are some that I almost always use on my tablet (because they benefit so much from the larger screen or the more powerful battery that they’re not worth using on my phone). But if something in the form factor of the Galaxy S 4 came with a god slide-out keyboard (like my current phone) and a really high-capacity battery, I’d probably want that instead, and I’d go ahead and do my ebook reading on my phone too.