Thai - we have all been saying for a long time that it is wasted revenue having that part of the stadium empty, for both big games and not so big games.
I am sure they could create some sort of barrier between away fans and Millwall fans for the smaller teams who only bring around 500/1000, if they both sat in the North stand. That will in turn create an even more electric atmosphere in that stand but the police costs would probably outweigh the ticket revenue.
There is a way around it, but it is a nice "safety net" to have that gap between opposing fans at The Den. Also, I think nearly every time a team goes into that bottom tier, the robocops and horses are called in to stop fans clashing.
Good grief, I thought things had moved on from those dark days (with the exception of Leeds, Birmingham, West ham, Forest etc). Real shame, as you say a better atmosphere could be generated. Maybe one day


keep it up your doing a great job pardon the pun 
