Cant see how it would be an advantage. You can only ever point it directly upwards to the zenith, and the sky is always moving - so how could they get sufficient time to gather light for photography etc. You couldnt track stuff for even a second, at the accuracies that are worked on these days.
Only thing I can think of is have the mercury spinning pointing up, and then somehow articulate the tube to point or track anywhere by cunning use of strategically placed servo controlled mirrors/reflectors etc. Then you are defeating the object and it gets heavier again.
Also spinning mercury, that must involve minute vibrations??
There goes your nice smooth mirror.
Naah, put that one in the "failed good ideas" basket.