I find it amazing that this even leaked out.
Granted, audio of celebrities flipping out is always a big seller. Remember how long we had to listen to Alec Baldwin cursing out his daughter on her answering machine? Bill O'Reilly going mental on his producers back when he was still pretending to be a journalist also comes to mind, although it isn't really in the same category.
The speed with which this came out might be what surprises me. A month might seem like a long time, but not when you take the scope of a major film production into consideration. On-set explosions usually don't make it out this quick. The Lily Tomlin and David O' Russel screaming match on I Heart Huckabees, for example, didn't make it out until after the DVD release. Of course, an independent philosophical comedy that went over the heads of most audiences probably doesn't compare to the latest entry into one of the most popular modern science fiction franchises still running.
The reason it was recorded is actually a bit mundane. You can clearly hear Bale say that he would leave the film if Shane Hurlbut screwed up one more time. So, the recording was sent to the insurance company in case he made good on his promise. Completion Bonds are no laughing matter in Hollywood, especially when you are talking about a mega-budget summer release like T4.
As for why it was leaked to the media, odds are that someone in the production wanted to get back at Christian Bale for being a complete dick about the whole situation. If there's one thing that people have an increasingly low tolerance for, it is celebrities that don't play well with others.
Of course, what it all comes down to is the opening weekend Box Office totals. So, what you have to ask yourself is, would a YouTube video of Christian Bale crushing puppy skulls under his boot heels in between takes on the set of T4 deter you from buying a ticket?