The idea of using progressively harder bosses - which is what you are really talking about - is nothing new. In fact, it goes all the way back to Wolfenstein...where the boss at the end of the first mission was a big blue guy with chainguns. You get to meet the same guy mid-level in later missions, with more difficult bosses at the end of those missions.
I see nothing wrong with that trend continuing, because it makes the game more challenging as you progress.
I also like the idea of being able to bring an AT-AT to it's knees. ;-)