"What happens when the phone rings?"
"So you could use a headset but that's then more kit/cost/complexity."
Headset is the obvious answer. I already use a headset with my phone, and holding the phone to ones hear is a hindrance while working anyway.

"And if you lose/break your phone, that's all your productivity gone."
Also true for laptops, tablets and similar portable devices!

"Why would you not just have a cheap laptop/desktop/tablet?"
Most people want/need a phone so if one can get a smarphone that can also replace a "cheap laptop/desktop/tablet" then one will save money.

"If all your stuff is in the cloud you can work on it from any device anywhere"
All my stuff is not in the cloud (I don't want my stuff there) and even if it is having it already on the device is much faster, cheaper and is not dependent on the availability of a internet connection.

"so why restrict your power to a mobile when you're sat at your desk?"
The power of most recent mobiles (e.g. Galaxy S3's quadcore 1.4GHz CPU) is enough for the computing needs of most people.