It's a bonus game in the sense that a loss tomorrow wouldn't change what would still be a successful 10 day/9 game roadtrip. Obviously, tomorrow's game counts just as much but it's a lot to ask against one of the best teams in baseball.
Wiemer has a .900 OPS against LHP. Frelick doesn't have a hit against a LHP since he hit that HR. Playing the percentages is always a good idea. Better chance of converting a run there with Wiemer hitting against a LHP and with a runner at 2B than if we were to give Frelick one more AB in the game (if he would even get another AB).
The only thing is he's quickly approaching his career high in innings and has been injured the last two seasons. But, obviously trying to win this game now.