There's a slight difference though. A tablet is meant to be carried around and an external keyboard kind of ruins this, though I admit there've been solutions to this which didn't completely suck.

A workstation is stationary, so mobility isn't an issue and usually the keyboard comes with the workstation anyway.

And on a sidenote, what you're basically saying is that it's easier to be productive when you have any means of I/O at all than when you haven't.

Doesn't change a thing about what those devices are focused on though.