OK, in general, a stiffer tail will give you more kick and stability finishing a turn, and more directional stability at higher speeds, but it'll be less forgiving. And a handful in bumps. Skis designed to jump tend to have balanced flex, with softer, springy tips and tails to launch and land, but firmer midsections for stability (Dynastar's Big Trouble being a classic example).
That said, I'd have to disagree that stiff tails are "preferred for Big Mountain." It depends on who you talk to and what brands they love. Stockli's crank through crud, pow, and probably concrete at speeds that'd melt your parka, and they are well-known for having relatively softer tails, that create a smoother finish, than many other brands. Gotamas have a pretty good rep for handling the backside, and they do not have particularly stiff tails. In fact, most twin tips have looser tails than most flat tailed skis, and if you haven't noticed, twins rule the backside now.
From the other perspective, a really stiff flat tailed ski is great for deep pow and crud if you're the type that needs to hit 60 mph at Valdez.