Metronomes do not do much except guard Tokyo Jupiter like sentries. It is unknown if they are actually fully fledged Dolems or not. They float in the sky and can defend by creating a sound barrier which they used to trap the RahXephon. It is unknown if they are even linked with a Mulian, though they most likely are.
One possible answer is that they are linked with the entire population's consciousness rather than one dedicated Mulian, and are a physical manifestation of how the citizens of Tokyo Jupiter feel they are safe and protected. This may be why they are not seen towards the ending fight, since the Tokyo Jupiter citizens can see the outside world and Mulian world and therefore no longer feel safe.
