The way I play my 0/21/40 build is that I take talents that allow flexibility between Shadow and Fire. That way all I need to do is a quick neckpiece change along with different potions and the right demon sacrificed to immediately change orientation.
Take for example Supremus. When I played fire against him, I noticed a stark drop in damage dealt. I tried the next week with shadow and kind of topped the overall meters for him. Then there's the Illidan fight where P2 has fire-immune elementals.
Personally I find the fire spec to be more fun. You still have to manage a DoT (Immolate) and the graphics are nicer. Plus I seem to deal more damage with it than as shadow. Hence why I'm kind of pleased with the new fire lock changes in Wrath of the Lich King
