They may play the ealier rounds with entirely their second squads, and fielding incompletely first teams for the final match. Although at times a club may go all out for a cup if it saw little chance of getting any other trophy that year.

I think ManU started the trend. They started fielding a younger second squad (including a young Beckham) for cup games in the early nineties, and the practice quickly spreaded.

After all, finishing, like, fourth place in the Premiership brings you a lot more financially than winning a cup does. Sad but people are more realistic now.