The difference is that the J/30 always had the range of head sails to choose from but just a signal spinnaker. I believe that the major concern is that by allowing a choice of spinnakers, we would effectively raise the cost of ownership for increased sail inventory. One way to preclude that would be make it so only a single spinnaker type is allowed during a regatta series. The downside being, a light air spinnaker may not make it in heavy air.

On a related note, I sent an email to PHRF-NB asking if there would be a rating adjustment if a J/30 were to use a 0.5 oz spinnaker instead of the 0.75 oz spinnaker. I'll post their reply here when I get it.