I'm new at sailing cat rigged and always used the jib as a bit of a "crutch" to help trim and steer upwind. I have been trying to learn to sail cat rigged on my new Taipan. It points very well in light air, but I find I need to sheet a lot less than I think to get good airflow, keeping plenty of twist and camber in the main. Anyway, in light air I can make better VMG with the jib than without.

But here's my real comment. Sure, as Pete said the jib really helps reaching. Well, in light air, I've been sailing cat-rigged with the spinnaker. I reach with the spinnaker and this seems to be very fast. Spinnakers we use are, of course, really gennakers; ie they are very flat. If I sheet the spinnaker hard, I can reach fairly high in light winds. Your real question is about light air, right. As the breeze picks up I have to sail deeper with the spinnaker to keep from capsizing. Recently I found myself in 2-5mph wind conditions. I was amazed at how fast I could beam reach with the spinnaker, even flying a hull at times. I have not done two boat testing, but it is hard for me to imagine eecking nearly as much speed out of those conditions reaching with a jib.