Or at least to the capital. I like how in Civ4 you can only get a global benefit of a resource if it is at least connected to your capital. Since Elemental is a kind of civilization-building game, I think it's reasonable that cities have to be joined - or as another suggestion, make a travelling time between them (or to the capital) before a newly created resource from a city is able to be used by other cities. 
What do you think, especially about the last idea?
Best regards,
Steven.