The problem with bringing in somebody in an emergency situation from another team is that they'll have almost no time (if at all) to take any snaps with the offense, learn the playbook, get used to the coaches... You're pretty much screwed and forced to use plays the QB was familiar with on his old teams, or just use him to hand the ball off all the time.
If you have a third string QB, he knows the coaches and at least some of the playbook, and has had at least some experience with the first team offense. Much less of a learning curve. Although if your top two QBs go down for any extended period of time, your season's probably over anyways.