Here is an outside bet, although it goes against the grain of my thought process, this new "2 tier" terminal is in my opinion only being laid out because someone has said they want it as they wont pay full handling charges for a full service terminal. Nobody sensible would do it off their own bat, so who is asking for it?
They seem to have mothballed half of the terminal to cut overheads, its clearly only needed a few times a week this year!
Why would it cost an airline more to use the terminal anyway? Its hardly the biggest airport in the UK.