The problem is not just trust. If everyone is running the same software then there is a risk of a serious bug that could take down the entire network. With multiple implementations, that risk is limited.
You mean bugs like back in 2010, when someone created 184 Bitcoins? Different implementations might indeed reject those blocks instantly, but if any of the implementations would have a share >50%, it still wouldn't stop it. One way or another, it would need to be fixed and trigger a chain reorganization, and I'm not even sure if it's better if different people see different blocks as the most recent ones.