At first sight it didn't worry me too much, then thinking about it...
(1) Any alcohol content in the fuel (and there usually will be some) and much of the water will be absorbed by the fuel - potentially giving problems in flight.
(2) If the above happens, how do you know if it's a drain system fault or effects of the fuel.
(3) I'll bet that without agitating the aircraft (which presumably means flying it) at-least some water will sit in a corner of the tank without making it initially to the sump and drain.
(4) It is a rather glib assumption that all aircraft will meet the modern 400ml minimum sump size.
Overall, I can see some little sense in the idea of the test, but the practice does leave rather a lot to be desired.
Incidentally I was very involved in the decision to approve unleaded MOGAS for use here in the UK. As part of the exercise, the CAA tested various brands of fuel by mixing 50:50 fuel/water, agitating it in a jar, and waiting for it to settle. In one brand they actually ended up with more water than they started - they never did get to the bottom of why, but it would really bugger this CASA test method!
Regards,
G