I believe that the main reason for the single ban is the lack of apron space.
Sheffield is certainly no worse than many other airfields regarding EFATOs etc. Even Berlin Tempelhof accepts singles and they are right in the middle of the capital city with no open ground anywhere near (completely mad in my view).
I wonder if SY's solution will be to open to singles but make parking prohibitively expensive?
Incidentally, I have landed at SY many times, and spoken to the controller many, many times in transit, and have found him perfectly clear. I wonder if people think how offensive they might be being to a nice guy who may well read this forum himself.