Okay, maybe that's going a little too far, but hear me out:
IMO, I think there is a good chance that Marrone has had his mind set on Manuel from the beginning, and here's what his plan may have been all along: To give Manuel a fast track training that he could not have obtained if he was named the starter 2 months ago. Rookie QBs face a whole new level of pressure going into the NFL, and Marrone is giving him doses of pressure to prepare. First, the training camp battle - this will give Manuel something to play for more than just "practice" - give him the mentality he's actually fighting for something. Same goes for preseason...if he's fighting for the job, it is a more similar mindset to a regular season game than just a "scrimmage" game. Manuel is under immense pressure right now instead of naming him the starter so he can breeze through camp and preseason without a worry, then get smashed in his debut.
Other people have pointed this out: If we let Kolb start all year, and he fails, then it leads to 2014 being ANOTHER rebuilding year with Manuel's first starting season. Coach knows we need to get the ball rolling.