In addition to United's 31 Premiership games so far this season, they have played an addition 22 matches (not including friendlies). 53 in total so far. It is impossible for players to play that many matches, so SAF has given his younger players a chance in the Carling Cup and FA Cup competitions. It is only right that he should stick with these players all the way.
In the great scheme of things, winning the League is a bigger priority than winning the FA Cup. It is more of an achievement, it earns more money, and gets the team into the Champions League. If a team wants to attract the best players in the world, they have to be in the Champions League. Just look at Manchester City, the richest Club in the world, and no decent players want to go there because they are not in the Champions League.
It might have been prudent to have had some stronger players on the bench to come on if the fledglings didn't have a result with say 20 minutes remaining, but that didn't happen.
There cannot be many football fans who have not been entertained by United this season.