I guess the primary question is whether there are things where there's value in making -sure- an attribute only appears once in a given object. The two uses (keeping in mind that I'm not sure what the problem domain for Querki is) that come to mind are unique IDs (where even if they're a secondary key, there are often good reason not to allow a given object to have more than one -- for instance, letting traversal work properly) and for linear data structures, next/previous.