Mxsmanic wrote:

Magnetic south isn't exactly the opposite of magnetic north on the Earth's
surface,

No you are confused. Magnetic SOUTH is the exact opposite of magentic
north you fool. The fact that the MAGNETIC SOUTH POLE is not exactly
opposite the MAGNETIC NORTH POLE is a different issue. The fact that
the poles aren't aligned with the arbitrary "true" datum just means that
the variation equations aren't as simple as they might be. This is why
we just resort to looking it up on a chart.

Early navigators used it because they had nothing better. Today there are
lots of things that are better.

Really, like what? Compasses are still damned reliable compared to
anything else. Cheap too. The only thing that presumes to do better
perhaps is GPS. And it's easier to program that to deal with the
magnetic measurements than to try to force everybody else the other
way.