Frelick isn't a bopper, but he has enough power to be around a 2-3 win player on average (which he was on pace to do last year). Lingering thumb injury really sapped him of his power last year and his ability to turn on inside pitches as he had done successfully in the minors.
With Mitchell and Wiemer, it's are they going to be able to make enough contact for the other parts of their game to even matter? Mitchell has an additional concern with his ability to elevate the baseball...
Obviously you have to give those guys opportunities on the chance they do figure it out, but I think Frelick will prove the best value in the OF at the end of the day. Not a star, but a well above average MLB player.