Seriously, if you're doing office work, get a typewriter. Just because you have a pimp desktop doesn't mean it should handle everything.
Seriously, if you're doing gaming, get an XBone. Just because you have a pimp desktop doesn't mean it should handle everything.
Seriously, if you're listening to music, get an MP3 player. Just because you have a pimp phone doesn't mean it should handle everything.
Do you see how ridiculous this is?
There are serious benefits and advantages to convergence: using one device for many purposes. Unless there is some reason why the hardware is especially poorly suited to the task, there's no reason *not* to do it. Supporting needless device specialization plays straight into the hands of the consumer electronics companies, who want you to spend $300 on a music player, another $300 on a phone, another $300 on a tablet, another $300 on an eReader, another $300 on a device to visit facebook, another $300 on a device to visit Yahoo, another $300 on a device to play games, etc. etc. etc.
A few exceptions:
e-Ink: acknowledged that it's superior, mainly due to battery life and readability in a wide variety of light conditions, for long-term reading of plain text. However, many people read large amounts of text on LCDs/AMOLEDs with no adverse effects.
External hardware for music production: acknowledged that skilled musicians can usually do a better job using actual instruments (whether MIDI or just a microphone attached to a conventional instrument) at music production. However, these microphones and MIDI sources can, and should, be compatible with general-purpose computing devices, including smartphones.
Physical keyboards: if you're typing something longer than a Tweet, it's so much faster and more comfortable to use a physical keyboard, even if it's just a small one. However, Android's support for bluetooth keyboards is pretty good, so that's not to say you shouldn't support physical keyboards attached to devices that normally lack them.
Actually, the latter two of those aren't really *exceptions* at all. They exactly prove my point. A general purpose computing device plus external accessories can be much more broadly useful in a diverse array of situations compared to a general purpose computing device that only supports phone calls and web browsing.
So then we have the following categories of devices:
(1) General-purpose computing devices. SHOULD have very robust support for external accessories to make it useful in a wide variety of use cases.
(2) Specific computing devices that serve one or two purposes. These are only justifiable if the hardware feature(s) are so essential to the task being performed that it has to be built-into the device, and it doesn't make sense to make it an accessory to a general purpose computing device. For instance, it would be silly to attach an e-Ink external display to a smartphone.
(3) Accessories. These don't have any real "smarts" to themselves, but they connect to a general-purpose computing device that provides some kind of value-added processing or data storage or streaming, etc.
Reducing the utility of general-purpose computing devices and selling more specific computing devices that are unnecessarily specific is an anti-consumer tactic.