Mary and others please prepare yourself for this one as the truth is a little confrontational :
... but why would anybody, for any reason, put a weight at the top of a mast? ...
Because other sailors without any engineering skills/background PERCEIVE that they will be uncompetive if such a thing is not done to some other crew despite the fact that the theoretical differences are too small to measure anyway.
Scientifically speaking there is no reason to suspect that a well designed F16 carbon mast is noticeably better (or worse) then the well designed lightweight Alu superwing section. However this doesn't prevent the bulk of the sailors to have a gutt feeling that says otherwise. For some reason people tend to trust their gutt feelings over verifiable science.
Take a look at an analogy :
Why would any person prefer 6 liter V8 diesel engines over the newest generation German 2.4 liter V4 diesel engines ? With the extra gear shift the last beats the first hands down in all aspect. From overall weight to road speed and pulling power to fuel efficiency. But still the gutt demands attention ....
Wouter