Let me me disagree with SH 100%, and I think this is a first!

I just booked a last-minute family trip to BGI towards the end of FEB., 4 of us total. I booked cheap Leisure seats, but courtesy of my ELITE status, I book the tix, go to seat selection, and yes, we are all seated together, and in either rows 12 or 13. The rest of the aircraft are jammed, no 4 seats together anywhere, other than, you guessed it, in the Elite-block section of the aircraft. This is a gift, and makes Elite status worthwhile, absolutely. As for giving away seats at boarding to non-Elites in the Elite section, well, I would certainly use the same method of load management if this were my business.
As for the 85%, and the idea that seats could be kept open as a perk to SE's or E's...are you kidding? We all know that the 85% is the average load, but that the flights we are on always loaded 100%, or are empty as Stephen Harper's head...Look at the YUL-YYZ shuttle...the days I'm flying, the peak hours flights are jammed, but the 11:00 pm, 11:50 pm flights run at about 50%, thus the average.
Your idea of blocking off middle seats until the back is full...makes perfect sense, meaning it has no chance of being implemented. In order to make such an idea fly, AC would need GA's, and all staff in general, to understand the concept of customer service, and what the drivers of true loyalty are. They aren't there yet...at least not most of them.

$0.02