The theory is that dropping the jib creates extra seperation between the kite and mainsail and helps make the kite more efficent. Some teams do it and others dont bother and its not really conclusive which is better.
There is a lot of seperation between the kite and jib anyway and the wind is always well forward of the beam so I tend to think its not needed but sometimes it does seem to work, but without proper two boating its hard to know for sure.