Comment 36 for bug 1460222

@Ed: Sure, here's a contrived example:

A service provider may have multiple tiers ("gold, silver, and bronze") for purchasing network bandwidth as well as latency. If a customer is able to purchase these levels separately, they may wind up with a port that is tagged for gold latency, but bronze bandwidth. While you can always mangle strings (e.g. ["latency:gold", "bandwidth:bronze"]), it is generally better to keep the scope separate from the value: [["latency", "gold"], ["bandwidth", "bronze"]] or [{"scope": "latency", "tag": "gold"}, {"scope": "bandwidth", "tag":"bronze"}]. This facilitates queries such as "which of customers purchased gold service plans?" and "what ports don't specify any latency tier".

Non-contrived examples (within and beyond networking) abound, but are left as an exercise for the reader :)