It's true that a compass on the device is very nice.

But I don't think that a bluetooth compass would be useless.

First of all:
You don't need a compass to follow a map. My old pocketloox t830 has no compass and follows the map by GPS. That is no problem as far as you are moving. Only if you are standig immobile the device loses its orientation. You don't need a compass to follow a map. For this application a compass is more or less useless.

A compass is usefull, if you are standing at a place and want information of what you see in a direction.
In this case there is no difference if you point in that direction by the n900 or by a bluetooth compass.
(I think even that a separated pointer would be more confortable. You can fixe your eyes on an immobile spot.)


The third case are games with a digital compass. You have to play them the bluetooth device fixed to the n900. That is no problem during a little game.




I think that in all those cases a bluetooth compass would be an acceptable solution. And this compass has even additional features:
temperature sensor, altimeter, independent gps logger, movement sensor ....
Perhaps those informations could also be interesting for some applications.