The main reason is that these older aircraft aren't fitted with autothrottles that can be used on the ground. The flight engineer sets the power watching the forward engine instruments (between the pilots). He may make several adjustments as the aircraft rolls down the runway - there is no way the Pilot Flying can watch the engine instruments while flying a successful take-off. On 747-400s, 777s etc, the throttles still move but the power setting (EPR) is in the computer and the precise setting of the engines is automated. The Pilot Flying will still feel the throttles juggling under his hand, but it's the autothrottle doing it, not the trusty flight engineer. The same applies in flight, the flight engineer used to reach forward every so often and change the position of the throttles, now the autothrottle does it. Bummer, I prefer three-person flightdeck crews.
The Sabena technique is an odd one, but obviously must work, or an airline of the stature of SN wouldn't do it! In normal (non-SN!) operation, the Pilot Flying, to absolutely clarify this, removes his hand from the power at V1 because that is the point at which it is too late to stop on the runway. Whatever happens after V1, the power is staying where it is cos you're taking off, alright?!
fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz