It's about the failure modes you anticipate. If a text file becomes corrupt a human can probably still make some sense of it. If a binary file does, you may well be completely out of luck. I mean try it for yourself, truncate a 4G core dump by a couple of bytes and watch GDB try to open it. Now imagine that all your system and application logs are vulnerable to this.
In a world where things are "fixed" by just blowing away a VM and spinning up a new one, you might not care but in that case why bother to log anything at all...?