A few of you mentioned the idea that the Stormtroopers take on a different and superior dramatic aspect with the knowledge that there are regular troops out there somewhere and the guys in white armor showed up here!
That's good enough for me and I think adds something to the game. Not to mention, modern military trends not withstanding, it makes sense. The Empire spans a galaxy and no doubt has millions of soldiers in its ranks. All of them Stormtroopers? Doesn't sound right to me. I see the various regional forces as mentioned somewhere and then the Imperial regulars assigned to the Empires various interests.