Completely new to the game, quick question.

Is there any compelling reason to play 12 year fully developed guys in preseason at all?
Are they rusty without playtime in preseason, etc?

they will gain conditioning if they get some play time. Also if you introduce new plays it can give them knowledge of the new plays. Otherwise it's not that big a deal. I try to get the conditioning up on my starters so they will be less likely to get injured in real games but i'm not sure how much it matters to be honest.

Good to get players experienced with the plays you run which is especially useful if you sign new guys In the offseason. Also, you want to condition your starters so that they don’t get hurt when they start playing full runs. You also get a sneak peek to how other teams might perform but that’s never a guarantee.