Beaufighter: why isn't the rear observer position usable?

[Igo kyu]

I am not talking about the Aussie Beaufighters when I say I want a 0.303, I know nothing about them, and am not an expert on Beaufighters in general. However, a gun firing backwards seems a useful idea, and apparently they were present in some British Beaufighters. There's a picture of what's said to be a Mk VIC showing a what appears to be a mg in the observer's position, at the top of page 29 of Fighting Aircraft of WW2 edited by Bill Gunston.