It's not about being bug-free, it's about being crash-safe. You should be able to crash your software - either by pulling power cord from the server, killing process or turning disk read-only (similar to what happens when it's full), and your database should still survive.
Data corruption like that should just not happen. You have your journaled filesystems, you have your raid, zfs pools, and whatnot; all that is worth nothing if your database software can just say "I have encountered an error during write operation and now your data is inconsistent, good luck". This is exactly what journaling / write ahead log / innodb doublewrite buffer should prevent.
There is an article "Crash-only software: More than meets the eye" on lwn.net if you would like to read more about it. The postgres fsync bug is also vaguely related to the same issue, also worth reading.