See the below article for more theories:
If the Eagles wanted to use the emergency quarterback – Vick in this case – in a first-quarter, goal-line situation, he would have to play quarterback, and McNabb couldn't return. If Vick didn't enter until the fourth quarter, McNabb and Kolb could reenter thereafter. Vick could leave and reenter as well, but he could not "participate in the game at the same time as either of the club's other two quarterbacks."
"Another aspect of the rule is sometimes misunderstood," Signora said. "It is a coach's decision as to whether a third quarterback will be used. The active quarterbacks do not have to be injured for a team to use its third quarterback."
...The Eagles opened this can when they assigned Vick No. 7, a quarterback's number, and openly considered him a "bona fide" quarterback. Reid was asked if players generally thought of as quarterbacks would be listed on the roster as such.
I always thought the starting QB had to be injured to bring in the 3rd QB. But once the emergency QB is brought in, the starter can't come back in. And what about our beloved A.J. Feeley, who netted us a 3rd rounder from MIA a few years ago? He's now #4 on the depth chart and he would be tough to let go given his experience in the offense and success in Philly, filling in when McNabb and Detmer got hurt a few years ago.