Miz is one of the few pitchers with intimidating stuff. It's good enough that he can get away with the occasional mistake. If he was more wild, Randy Johnson would be a good comparison, but Randy didn't really become great until he hit 30. So far this year, Miz's control has really taken a step up, but we are dealing with the ever popular small sample size.
On other factor with extending Miz is that nobody knows what is going to happen with the new CBA. That might swing the economics in favor of the Brewers being able to sign him. Or not. Hoping that MLB will do something that will work to the Brewers favor often feels like hoping Santa Claus will bring you a winning lottery ticket.