I don't think Mary doesn't want to give the young guys a chance, he has tried Host and Sele. Since his early days coaching, he always spoke about having experienced guys hence we got McCrone.
As Mary matures and with his position now secure for the next 2 years, Mary has a free shot to blood Field and Duffy. If they don't perform, he has the excuse of Origin Affected teams, yet he can claim that this aims the long term good of the dragons.
If they work, the pieces of the puzzle become clearer for next year.
Also, Mary needs to keep people in the same positions week to week, particularly our much criticised backline, has regularly changed every week for the past couple of months. The forwards are less of an issue, however the changes affect our fluidity in attack, and our positioning in defence. Hence, moving Nightingale to Fullback changes the composition of our centre wing combination again. I would rather keep Nightingale on the wing and try either Mann at Fullback or Duffy. At least people are playing their preferred positions.