The Sacrifices of Portablility?

hackwrench asks: "There is lots of talk about writing portable programs, but this pursuit has resulted in a lot of processor features going unused. One example is being able to write a program that purposely uses a combination of 16-bit and 32 bit. I know there are arguments that writing solely in one or the other is a performance advantage, but what are the factors involved? Is the slowness of such a combination inherent in its design or is it a result of current hardware. We are beginning to replace systems and programs designed primarily to run in pure 32-bit mode with systems designed to run in pure 64-bit mode, so I ask: Is such purity really worth it?"