OK, Iain I have to say tha MD-11 is right. Flap deployment has only to do with speed. It doesn't matter if you're a Cessna or a 747. Because Cessna's have straight, non-critical wings high lift devises aren't ABSOLUTELY necesarry.
Ask this, if you landing your Cessna on a 1000 fot long runway and you're a bit too low, are you gonna use your flaps to land? You better because otherwise you're gonno go off the other end. That is the MAIN purpose of flaps. Of course there aren't many 1000 foot long runways and a Cessna is so light that use of flaps on landing only makes a couple of hundred feet difference, so here they're mainly used as speed brakes. But look at a jetliner. The supercritical wingshape they have means that tyey're most efficient at high speeds. When landing a 747 with no flaps means to come in at I would guess about 220 knots!!! Flaps increase the lift capability of this wing. And that is it. If ATC gives a 747 a very early descent and asks it to maintain f.e. 170 knots. It better use flaps because otherwise it'll come crashing down.
As for take-off Jetstar is right. A 747 at 650000 lbs lifts off at 150 knots with flaps at 10, but only at 140 with flaps 20. Thats a whole lot of difference at that speed.