[Roberton3]

...the backups for Luck, DeCastro and Martin essentially never left the sidelines.

That's not quite true.  The OL backups played quite a bit in packages with extra linemen.  So they have game experience.  But playing as an extra lineman isn't the same as playing guard or tackle in a more standard package, particularly when it comes to blocking schemes and blitz pickups.

[Roberton3]

...the backups for Luck, DeCastro and Martin essentially never left the sidelines.

That's not quite true.  The OL backups played quite a bit in packages with extra linemen.  So they have game experience.  But playing as an extra lineman isn't the same as playing guard or tackle in a more standard package, particularly when it comes to blocking schemes and blitz pickups.