Muggles' Guide to Harry Potter/Magic/Point Me

Overview
The Point Me charm, or Four-Point-Spell, makes a wand turn so that it points north.

Extended Description
This spell was used by Harry in the Third Task maze in Harry Potter and the Goblet of Fire. Knowing that he had to travel roughly north-west from the entrance of the maze to reach the center, he was able to use this charm at several points in the maze to correctly select a direction to travel in.

Analysis
Basically, this spell is a magical equivalent of a Muggle compass. it is, however, slightly more useful as a wizard will have his wand with him all the time; there is no need to carry a compass in addition to the wand.

Questions

 * 1) What would the disadvantage be, to a wizard, of carrying a compass? Is there some reason, apart from the small effort needed to carry an additional item, that a magnetic compass would not work?