This kind of thinking is very much applicable in this time and age where smartphones are becoming smart devices not limited to only 1 task: phoning. They can do more and more, and the general public wants this. They get more features for the same price a device with less features cost a few years ago.

There is this turning point where the amount of money invested in a dedicated device for a specific purpose (example: e-book, photographing) is worth it. Sometimes there is no other solution available. While that might be software related (missing driver or bug in driver, no port available, proprietary protocol) I usually see the reasons are hardware limits of the all-in-one device.

For many people a camera on a phone is good enough, but sometimes a SLR is really required for professionals. I imagine for some people a dedicated GPS is required. If you also take the screen into account you cannot simply optimize for stylus input, finger input, keyboard input and then make it good for browsing, e-book reading, both inside and outside in direct sunlight. Tough decisions are made during hardware design.

So we see people who'd like additional devices besides their smart device. Usually its professional items which are expensive, not casually used by the average individual. They don't always carry the additional device but my oh my, every photographer has this situation where they just wish they had their SLR with them. Having the smart device with them allows them to at least have the option to make a photograph at all. Ofcourse the quality is then not acceptable, but there might be situations where the fact you have a picture is more important than the quality. Imagine having a GPS device, as basic and archaic as it is, it got you to a point where you weren't lost anymore.

I believe also that eventually we are able to have a touchscreen more flexible to be applied. For example by making it detachable, or by having the smart device able to dock easily, or by having it able to talk over HDMI or BlueTooth. There is no reason why every device must provide its own input abilities which directly interface with the human instead of indirectly via other devices.