turnandburn I'm sure that this message has been devised for both the rasons that you have mentioned.
It is not unheard of to have a parallel r/w fed in th box and to then mistakenly line up on the other runway and even get take off clearance undetected by ATC or flight crew. Specially in low vis conditions.
another example would be to set up for one r/w at the gate and halfway during taxi the controller offers another closer r/w which will help expedite departure and the crew accept it in a hurry to get airborne asap.
I know all these things sound so silly but providing the fmc with this safeguard is in line with the same logic as adding "landing gear....down" in the landing checklist. It's so obvious but what if there's that one in a million odd that one day someone forgets it. And it has happened.
Regarding different GPS datums, it happens to us regularly in Shanghai which is the only chinese place we fly to. But from reading other posts on this thread, it looks like it's common all over china. Boeing has sent out a bullietin for the 777 to handle such situations.
cheers