I flew years with 3 pounds of lead shot secured in a bag at the top of
the vertical fin above the battery cover (removed the black foam to
make more space). My local authority was ok with that and I never had
a problem, but also never ground looped.

Ramy

The bottom line is that cantilevering that kind of weight on the top
of the tail is just not something that the aircraft designers and
engineers anticipated. The structure wasn't designed for it - so just
because it has worked doesn't mean that it won't fail in the future,
or that it works on all gliders.

Disregarding structure, there are performance reasons to keep the mass
concentrated down in the fuselage. The closer to the center of
rotation that you place some mass, the less force it takes to put that
mass into motion.

Here's a thought-experiment: Imagine rolling the aircraft. Imagine
looking at the airplane from behind as it executes a perfect aileron
roll. If you put the weight down low in the tail-boom, the rest of
the tail rotates *around* that mass and it doesn't have to travel very
far or move very fast. If it was way out at the tip of the tail, it
would move through a much bigger circle over the same time period.
That means it travels farther and has to move faster. It takes energy
to start and stop the movement of that mass.

In short: Handling is going to be less crisp and it will be harder to
make subtle attitude corrections with a bunch of mass out at the tip
of the tail (or the tip of a wing, or any extreme end of the glider).

Take care,

--Noel