[Jeff Lee]

James is correct, you better check with your DD. I just looked at my '08 rule book (and I doubt anything has changed in this regard but check the online & current version). Under window net, it says "mandatory on any full bodied car required by the rules to have a roll cage".
When I look under both Stock & Super Stock, the rules define which classes or ET require a roll-cage but allow a roll-cage for slower vehicles. There is no reference one way or the other that a net is required or even optional. So basically one could argue that since the rule is not clear, having a net could be an infraction in a class that does not require a roll-cage.
The rule book is full of ambiguities. For example, the rule in paragraph 1 above states a net is required on any full-bodied car..." So I guess trucks don't count? Obviously they do. But somebody with a truck might make that argument one day.
I would still contend that roll-cage = net even on a 15 second car because you have elevated the chassis to the level of a roll cage so you have the net also. Same as I said about seat belts. Once you elevate the car to the level of aftermarket SFI belts (even on a 15 second car), you buy into recert or replacement every two years.
I'd be willing to bet that if I asked NHRA for clarification on the net issue in a slower car with a cage, the net would be mandated. But don't worry, I'm not asking!
But why would one want a cage and not a net?