Mark, here's a scenario!
The wind is blowing 15 knots and you are sailing an A-class upwind with full depower on the main (you are trapezing in 6 knots of wind on the A-class). Then add a 5 m2 jib which gives you more stuff (you don't need) to drag around. Will this be faster??

Classes with only sail area limit goes uni rigged. A-class, C-class, F40 (they had to have a very small jib for class reasons). Above the 40 feet limit there might be structural reasons to keep mast height down and add a jib to get more area.

There where some A-class boats in the beginning of the development of the A-class that split the sail area in a jib and main but the uni rigg was faster.

/hakan