I don't think Aluminum is valuable for Allomantic reasons, it's valuable for the same reason it used to be on earth, it's somewhat difficult to extract from ores (My understanding is that it usually requires electrolysis or extremely high temperatures) I'm sure that in AoL it would be even more valuable than that due to it's protective abilities but generally it would be about the same as on earth. Atium would never have been available to the public in large enough quantities to the public (Also this would have destroyed many plot elements in the book, since Elend and Vin would have definitely had Atium in WoA) So yeah, gold was probably used for the same reason it was on earth, it's rare but not prohibitively so, it's also possible that TLR chose this deliberately as if atium was everywhere, the nobility would have discovered Seers as would the Skaa which would give both a lot more Allomantic power than he wanted.