To be fair, most PL teams do that already - it depends to a large extent on how keen they are to win it. As shown by the match at Anfield last week, Klipperty put out a team of kids v Arsenal, who replied by matching them and lost on penalties!
It's obvious City enjoy the competition, but tailor the team for the occasion. It's always a mixture of youth and experience, plus the odd player returning from injury for playing time. The nearer the final, the stronger the team, but with the proviso that there will still be a youthful contingent involved.
As the prize for winning it is a place in the Thursday Night Cup, I'm amazed that most teams don't/can't be bothered putting out a competitive side, however it is made up, though I do note that a team in Stretford almost have the first team out on the cup match nights. (I suppose it shows how much they like the Thursday Night Cup)