Well, the idea was you paid to get the compass rather then having one floating randomly in space. And the compass drifts more if you use a cheap metal, as well as spins around if a rock contains iron or more lodestone. So you can tell whats in rocks before hand. Although you would probably smash it anyway, for the ore inside.
Who knows what you could do with aluminum. Build rockets, aircraft... Well, in game, you could probably make light-weight map cases or something, for when items actually start weighing something (press o. There is a section for max wieght carryable).
Hmm. Maybe a random metal bar plus glass could make a spy glass? And depending on the ore used, you get better magnification, and its more clear.