Buttery skis are fine for doing butters. However, if you are hitting large features or skiing a lot of pipe, the extra stiffness is very helpful as it provides support, and the skis won't wash out on you when you don't land perfectly.
People have this odd notion that all park skis should be complete noodles, but if they tried something stiffer, I think they might notice that it makes it more enjoyable on large features.