If the Tigers were targeting Barnhart one way or another, this was the best way. I'm guessing the Reds would probably not have exercised the option, making Barnhart a free agent, meaning we would have had to compete for him with a few other teams. That would have driven his price up to eight figures and multiple years, and we may still not have gotten him.
This way, we get him for one year, which is the important part, and at a price we can certainly afford. He may hit adequately or he may hit horribly, but either way, he can handle pitchers and frame pitches, and for a year during which we're not due to seriously contend, that should be enough.