The cons of Justin Herbert not playing in the preseason:
The biggest thing here is game-speed reps. Justin Herbert is not getting any sort of practice before the season to get into true game speed. Yes, Herbert jumped into a game with 10 minutes' notice last season so it might not matter, but it is a factor.
This would be more concerning if Herbert was a rookie but he is not. He definitely understands the speed of the NFL game now. That being said, this is a brand-new offense that he is jumping into without any sort of test against other defenses.
The main reason why Staley seems to be comfortable making this decision is his belief that the practices are truly replicating game-time situations as close as they could be. While I definitely think the Chargers are doing their best, I do not think that is possible.
Yes, Staley is a defensive wizard and is going to be throwing everything he has (besides tackles) at Justin Herbert to prepare him for the season. But there is still a difference between facing your teammates every single day and facing a defense that you don't know the tendencies of.
As great of a defensive mind as Staley has, he cannot completely replicate what other teams are going to throw at Herbert. Every team has different schemes, different athletes and different strengths.
It might not end up mattering, and as I said, I agree with the team's decision to rest Justin Herbert.
That being said, playing him for 2-3 drives over a three-week window is not the riskiest thing in the world and if Herbert has a slow start to the season we might know why.