Why would they bring someone in for camp? When was the last time they had 4 QBs in camp?
2009, when they had Brady coming off of surgery, Gutierrez, O'Connell, and Hoyer. Gutierrez was cut early in camp and they signed Andrew Walter.
They would bring someone into camp because they're not going to just hand a rookie the backup QB spot for the first 4 games. He will need to earn it; otherwise he'll be QB3.
- Garappolo as a rookie competed with Mallett in 2014, Mallett wasn't traded until rosters were finalized
- Mallett was the #3 QB in 2012 as a rookie
- The aforementioned 2009 when Hoyer competed against 2 (really 3 guys) and earned the spot
- Kevin O'Connell was the #3 QB in 2008 behind Brady and Cassel, and when Brady got injured they decided to bring in a (non-rookie) 3rd QB in Gutierrez.
- Cassel as a rookie was the #3 QB behind Doug Flutie in 2005
The reason you'd bring the QB into camp is to give him time to learn the playbook. Now, another option could be what others have mentioned -- see how Brissett does, and if they're not comfortable then bring in a Patriots retread (via waivers or late round pick trade) who already knows the playbook like Hoyer, Cassel, or Mallett.