These WRs aren't world beaters. They're not open all day, even if you give them time. Better Book threatens to run and make the defense respect his legs, it opens the possibility that a guy comes free underneath as a defender moves up to chase Book. If he stays home, Book can take off. If Book sits in the pocket and no one gets open then he has more ground to cover with his legs.
I’m not sure what play you were watching but the defender wasn’t covering anyone accept Flemister coming out of the flat. If you run at him, it does the opposite. The defender does not have to choose. He waits for Book to run at him and tackles Book. Just like what happened. He needs to create space. If the defender comes forward across the LOS, then Flemister is open. If he doesn’t then he stands there and has time to deliver to someone else. This has nothing to do with the WR’s. It was bonehead play that served no purpose. That’s it.
Let me just say, it’d be completely different if that defender was the option defender in an RPO, like how Mike Norvel has his QB’s press the LOS horizontally. But this wasn’t that. It’s a broken play where the best thing Book can do is keep the play alive until there is a throw to be made. Instead, he just ended the play himself. And this isn’t a unique situation for Book. He doesn’t this a lot under pressure.