I won't get too in-depth with evaluating the move, since it likely won't end up making much of a difference this year. What you need to know is that Carroll will be 28 this year, hasn't had much success with the bat, but is supposed to be a good defender. I thought Bernie's Crew did a great job of summing it up.
My first thought, though, was to wonder about the status of Corey Hart. If this move is being made for Triple A depth, it may mean the club is keeping more outfielders on the big league roster than they were originally planning to keep. Hart has previously said that his target date to return to action and still be ready for Opening Day is tomorrow, March 24. It doesn't look like he's going to meet that deadline. If Hart is going to start the year on the DL, one guy who was slated for Triple A (whether it's Chris Dickerson or Brandon Boggs or Jeremy Reed) is going to make the big league club, leaving a hole in Nashville. Of course, it doesn't help that Logan Schafer needed thumb surgery and will be out for 4-6 weeks.
In the end, this move isn't all that different than when the Brewers signed the likes of Chris Duffy and Norris Hopper in the past. There's no sense in getting too worked up over it, because if we ever see Carroll in Milwaukee, odds are we have bigger things to worry about.