From a pleasure sailing perspective, you can blast around the water on one hull when the wind is blowing or, when the wind is less than 10 kts, you can still go sailing. It's not as fast, but you can still get out on the water and not just bob around.

Just think about the design behind cats and monos. For a given length the boats weigh about the same. The cat can handle more sail area and can remove half of their hull area from the water.

Then, there is the cool factor. Cats just have that extra nostalgia. You will also notice that cat sailors are always ready to help you. Even if you are racing against them they will still help you tune your boat. You will not find a friendlier group of sailors anywhere.