Would be nice to the views of an expert on this but when it comes to players feeling tired towards the end of the season i think people seem to misunderstand what that means sometimes. For me it doesn't mean that they can't get out of bed in the morning and even if they do they feel knackered after 5 mins of a match.
Premier League footballers need to play to the peak of their ability for every minute of every game, and to do that you need to be as fit for each minute as possible, and it stands to reason that even the fittest of athletes are going to see an impact on their fitness levels the more they play.
Sure, they could easily play 50-60 games a season better than the vast majority of us, but the more games they do play the more chance that they cannot reach optimal performances for the whole game, they might lose a bot of sharpness that means they're seocnd to a ball, they might lose a second of thinking time that means they make the wrong pass.
So with that in mind yes he should make use of his squad more, they're his players he bought into the squad, he should use them. Of course Moyes has his way of doing things (sticking to the same core group of players as much as possible) and he's done pretty well. But when using a small group of players the tiredness explained above has to become a factor, we do have a habit of having 2 very differing halfs of seasons.
We have had games in the last year where we could have subbed certain regulars off when winning relatively comfortably. As much as we'd love to be 8-0 up at half time it doesn't happen like that but we have had games where we've had 2-3 goal leads, players could of been rested then, players can be rested for odd games against lower opposition without compromising the team.
It can be done but i think its a case of Moyes doesn't want to rather than he can't.