So I just learned, and posted a link to Charles. It says the reason is most teams only carry two QBs on their 45 man roster. The intent is that teams can bring in someone who has some idea about the position if the first and second stringers are injured. As you say, there is no requirement that they be injured but that clearly seems to be the intent.

>Yes, there is such a rule. Called 'The Third string Quarterback rule.'
>
>Once you sub in your third string QB, your other QBs become ineligible to play again in that game. The team has to designate who the third string/emergency QB before the start of the game.
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>Never understood why, but it is there.
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>
>>There is no such rule. Anyone on the roster can go into the game.
>>
>>Sorry about the Hawkeyes game on Saturday. (I'm not). I hope they bounce back against Michigan State. Northwestern had them and let them get away.
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>>>Wow, what a dumb rule if true.
>>>
>>>>If I'm remembering right, that's the rule. That's why he's called the Emergency Quarterback.
>>>>
>>>>>What? Seriously?
>>>>>
>>>>>>For lurkers, a Brady rule would be good, but it's the "Emergency Quarterback" rule that says the 3rd string guy can only play if the first two quarterbacks are injured.