Also, by using only your jib you move the CE (center of effort) of the sailboat forward while the CLR (center of lateral resistance) stays the same. This usually results in a lee helm that always wants to fall off, so in order to make any way going upwind you always have to steer hard up wind, which further slows your forward progress.
If you balance the forces of the sails (CE) by using both the main and jib then the CE is usually slightly behind the CLR which results in either a neutral or weather helm. Even using a heavily reefed main with a jib will move the CE aft and behind the CLR which gives you weather helm. Of course it is best to have them as close to balanced as possible so you use less rudder which acts more like a brake to boat speed.
Most of us instinctively know this (and I know that David certainly does) but here is a link that breaks it down a bit more scientifically then I can: Helm Balance – Center of Effort, Lateral Resistance, Centerboard, Mast Rake - Waves « Jordan Yacht Brokerage
Good discussion btw.