Terrible analogy:The way I see Mary approaching the experience vs juniors scenario is sort of like a leading hand on a contruction site. If the project is running behind would you rather an apprentice who is out of his depth or a halfback who has played in a finals series and has 140 odd games under his belt.. I know it's not a great analogy but you can see where he's coming from. He knows what he's going to get and I still think McCrone does his job well but his kicking game does hamper us considerably. Same thing with the fullback position. The game is as tough and aggressive as it's ever been and if you throw a 80kg kid into it week in, week out, he will break. Benji missed years of his career from injuries sustained in his late teens to early twenties. Even Bevan French has missed half this season and it may only get worse for him.
1) We arent running behind schedule, in fact we are well ahead where we thought wed be.
2) You aren't throwing the apprentice in alone, he will be surrounded by seasoned workers who will make his job easier.
3) The apprentice, while quite green, may have better skills than the old guy.
4) When everyone in the team starts to get complacent and construction slows down a bit, sometimes the energy and skillset of a young bloke can help lift the place, even if it's just the change to shake things up.
In addition, no one is suggesting to throw Dufty or Field in for the remainder of the season as you are correct, their bodies are not used to it. However, in a 1 off game to cover for origin, there's absolutely no reason not to play them. Problem is that in Mary's mind, it's better to play a mediocre player who we know will be mediocre than a guy who hasnt proven to be just as mediocre.