They want to win ballgames, but they are aware that if they want to retain free agents, they can't crap on a veteran with a big contract. It's very easy for guys on chat boards to say "play this guy over this guy". We're not paying the salaries. Salary has a very big impact on who plays or who doesn't. There is no "we don't want to win games". We don't own the team and don't have a say.
Zim has been playing better the last 10 days. If he's able to keep it up, he'll have some solid numbers at the end of the season. I am a Zimmerman fan, and will forever appreciate the great work he did back in 2006-2011 when he was the best player on the team. He was the only reason to watch during the 100-loss seasons. However, the situation simply is that he's an average first baseman at best these days, not a Gold Glove/Silver Slugger 3rd baseman. It's not his fault his shoulder started going earlier in his career than it should have, but the reality is that an average-fielding first baseman, who bats right-handed, and is streaky at the plate is not what you want on a championship team.
Regardless, he has one more season after this on his contract, and unless he's on the DL, he's going to start most of the games.