In the early days of mobile computing, the dominant vision was to get your application suite on "any device, at any time". Single purpose devices such as SMS messengers, email-only messengers faded from popularity, largely replaced by mobile gadgets that could, at least in principle, do every thing that a good old fashioned desktop computer could do.
Operating system visions started to pop up everywhere aimed at enabling a single user experience across a suite of productivity applications, regardless of form-factor, weight etc.
Things (as ever!) have turned out a little differently. Particular form factors e.g. smart phone, tend to be used as the *primary* device for a subset of the users full application suite. Moreover, many users like to use multiple form-factors *at the same time*.
Some examples from my own experiences. I can do e-mail on my phone but I choose to do it on my desktop most of the time. I will do weird things like e-mail myself when on-the-road, using a basic e-mail sender, to essentially put myself in my own in-box. (Incidentally, my in-box is my main daily GTD focus. I can make notes to myself on my desktop but I tend to accumulate notes on my smart phone. I keep my note-taker-app open on the phone even when I am at the desktop computer and often pick it up to make notes.
I can watch Youtube videos on my desktop but tend to queue up videos instead and then pick them off one-by-one from my smart-phone, trying to fit as many of them into "down time" as I can. Ditto with podcasts. I have a TV that has all sorts of "desktop PC" aspects from web browsers to social media clients but I don't use any of it. I prefer to use my smartphone (sometimes my tablet) while in couch-potato mode and will often multi-task my attention between the smartphone/tablet and the TV. I find it increasingly annoying to have to sit through advertizing breaks on TV and automatically flick to smartphone/tablet for ad breaks.
I suspect there is a growing trend towards a suite of modalities (smartphone, tablet, smart TV, smart car) and a suite of applications that in practical use, have smaller functionality overlaps than the "any device, at any time" vision of the early days would have predicted. A second, related trend is increasingly common use-cases where users are wielding multiple devices *at the same time* to achieve tasks.
Each of us in this digital world, is becoming a mini-cloud of computing power hooked together over a common WIFI hub or a bluetooth connection or a physical wire. As we move from home to car to train to office and back again, we reconfigure our own little mini-cloud to get things done. The trend towards smartphones becoming remote controls for all sorts of other digital gadgets is accelerating this.
I suspect that the inevitable result of all of this is that application developers will increasingly have to factor in the idea that the "user interface" may be a hybrid mosaic of gadgets, rather than any one gadget. With some gadgets being the primary for certain functionality.