Knowing the gameplay, I can't really imagine why a duck would even be needed.
For most 2.D games, its kinda a given.
People should remember that this game is going to be played in a 2.D format and that having it play similar to a 3.D game (such as only using a analog to move or not being able to duck) is just weird and foreign to a lot of people.
It would be like taking ducking out of 2.D Sonic,Mario,or Metroid games.
Ducking has been pretty much a given for pretty much every 2.D Castlevania title.
if MS really is trying to win some of the "classic" fans over, why take out such a simple ability?