That's quite right. Either all on or all off. If wx is CAT 1. Minima, you can still fly a manual approach. According to Boeing if you fly manually you need to disconnect the A/T as soon as the A/T is off. (either by swtching off the hard switch or by depressing the speed mode, both is acceptable, but be very careful of the later, because in the ARM mode you still have low speed reversion (protection) from the A/T! If you do an extended flare the throttles MIGHT come up very slowly during the last 5-10 feet (if the flare and the speed reduction is very smooth) and if you don't notice it you are floating along wasting valuable rwy distance...) If it's below CAT 1. you're doing an autoland anyway so might as well have the A/T on for the landing. And finally if manual landing in the MCP SPD mode: again be carefull, the A/T will announce RETARD and it will close the thrust levers at 27 feet regardless if it is coupled to an A/P approach or not... And that might end up in a less than desirable landing in a heavy -900 in 35 degrees centigrade...