Please avoid sensationalist headlines. I don't know why this is being described as a "feature" here, but not on GitHub, nor why it causes "unreadable" logs, nor why it's only in production. Context?
Well it is a deliberate choice, calling it a feature may still be a stretch.
The log becomes unreadable because if you have many Rails instances (Like 25 unicorn workers in my case) writing unbuffered in one log file it becomes a holy mess. This is happening only in production because you usually don't have multiple workers in development and you also don't have hundreds of requests per second.