At the right price, he provides a ton of things. He already knows the system; he is a solid pass blocker, route runner, and receiver; and a decent though flawed runner (esp in the zone system), and he is a great locker room guy.
To me, you simply can't trust Herbert to do anything other than carry the ball at an NFL level. You certainly cannot trust the vast majority of rookies to be able to pass block and run routes. With where Fields and the org are at, to my mind you absolutely need to have a guy who know is going to hit his blocking assignment and catch passes back there. I also think that the Bears will and should value continuity in a year where there will be so little of it.
I admit I just admire the guy as a person and as a player, and so that may cloud my judgment here. But the good outweighs the bad at the stage of development Fields and the Bears are at imo. I'd say at $20m/3 years with the last year essentially not guaranteed, he's worth it.