One of the main arguments for hardware keyboards was you could type without looking. I don't really look at my phone keyboard when typing, I roughly know the spacing of the letters. Plus auto correct is really good at this point, so when I do make mistakes the phone usually just corrects them.
The only use case I could see for this is if the keyboard had control/alt/esc keys - in that case shelling into a machine on my phone might become slightly more efficient than an onscreen keyboard.