GASA,
Call me cynical, but I think you've hit it. If I remember correctly, one of the contributing factors in the Uberlingen accident, was 1 controller doing 2 jobs. The 2 747s at FL340 over NEVIS a couple of years ago, might not have got so adjacent, had the 'planner' been in the vacant seat.
The more eyeballs you have, looking at the radar......Sorry 'situation display', the better, and with 30% more aeroplanes forecast, you'll certainly need more eyeballs.