I can think of a few reasons for having a back up of backup for such essential items as a compass.

There have been times where I am more then 50-60 offroad miles from the nearest village or town. If I were to somehow lose or break my main compass, I would not be so certain that I would want to continue solo with only one compass, especically for hikes that may take me 2-3 walking days further away from the trail or area starting point.

Also when there is group travel, it is mandatory (no exceptions) that every person have a compass, amongst other personal survival gear. If by chance, someone forgets their compass (it has happened) then at least the trip is not worried with the burden that there is no spare compass to loan. This may seem like overkill or over thinking, however the one time that a person in a group does not have a compass, then it is this person who gets separated from the group and gets lost. Furthermore as the most experienced person in the group on many trips, I am charged with the responsibility of ensuring that everyone who may have less experience and travels with me has the requisite gear to ensure that their safety and well being is thought of before we head out.

As for the mention of creating too much gear and getting bogged down. My three compassses stacked together are about 1-3/4 times the height of a deck of playing cards and not much longer or wider and probably close to the same weight as the same deck of cards. I don't think that this extra bulk and weight would bog down any person, regardless of their physical condition...