Compass installation trouble

There's compass sensors in the Air Glide
sensor unit; so the Installation Manual
contains complete instructions on
required distances from magnetic
sources:

http://www.air-
avionics.com/air/index.php/en/customer
-support/air-glide/39-sensor-unit-
installation-manual/file

Here in Canada a magnetic direction
indicator can be used instead of a
compass in gliders. That may include a
flight computer moving map with a North
arrow.

So why install a compass?