O.K., here's my take on it. It's all about required power in conjunction with pointing ability.
Monohull: Weight is high, righting moment is low. This boat needs GOBS of power down low to drag it through the water since it is so dramatically heavier. Aerodynamic efficiency of the sailplan is a little less crucial since the speeds are lower. More power down low relates to less healing moment and better grip in the water if you can keep the keel closer to vertical. Therefore, if you get your power down lower, i.e. with a sloop arrangement, you can put more boogy in her dance and still point higher. Though they can sail less high with them, Genoas are used in light air simply because a blade jib just doesn't pull hard enough. The tradeoff in pointing ability for power is worth it because of the lack of wind there would be extra righting moment and they need the additional oomf. They need to use all the righting moment they can at all times. If you put a uni-rig on a monohull and attempt to carry the same amount of sail area, while it's efficiency can be higher and the power about the same, it's healing moment will be much higher and it will loose it's grip in the water.
Enter the catamaran: Weight is low. Righting moment is high. Keep in mind that most the uni-rigs (A-class, 18HT, etc.) are working with a box rule sail area. They take what sail area would be in the jib and add it to the main. Now the main is taller and with more sail area than a main sail on a sloop rigged cat. It's also designed with a very high finesse ratio (more efficiency through the air) and since we have better control over our righting moment we can manage the taller main. A jib is draggy and much less efficient because it presents a lot of windage that a main alone doesn't. However, just rolling up your jib doesn't make you point higher because you just lost 20 to 30% of your sail area, power, and speed to present lift to the boards. If I took my Nacra 6.0NA, built a new taller mast and added the jib sail area to a new bigger mainsail, it would point like no other because it still has the same power but a lot more efficiency. (although it probably wouldn't be very controllable!).
Downwind, I don't think anyone disagrees that a jib is an advantage because of the slot effect.
