hm. scarcity is a better reason to have spices as "luxuries" in the modern age. The amount of resources is limited, thus allocation of resources is necessary to allow people (or in this case civs) to make the best out of their scarce resources.
This being the fundamental problem of economics, I can go on and on about this for a while if you like - send me a PM
imHo, the resource model of civ uses the fact that resources are scarce in a good way - at least given the engine of civ3. Thus the fact that there is a limitation to luxury, strategic and bonus resources is justified - from the concept of scarcity alone.
Trade is the only necessary concept to adopt to try to gain more resources - if you have a surplus - trade it, and you might get some of the resource(s) you lack in return
Edit: The term luxury is used on goods that are not necessary to survive. Ask someone who is experiencing starvation if he or she thinks of spices as luxuries, and get a reply....
It might not seem like a luxury, but it really depends on the situation of the consumer. Poor consumers may find spices to be to costly (and since one can live without them), may regard them as a luxury....
just my NOK 2 (1USD=7NOK )