DISCLAIMER: All views are considered my own and you should not draw any conclusions on associates.
I used to think that all logs should go into the MySQL DB or whatever you want, but since my internship I'm starting to think that log files are the way to go. My aversion to writing to log files come from wanting to write to a log file immediately, but you don't have to It is probably best to keep a buffer zone in memory where the log file is written to after a certain quota or time. Then you can grab something like Splunk or some other cool log analysis tool to grab these log files and aggregate them for analysis!
There are things you'd still want to keep in the database though such as failed logins to keep out/lock accounts then you can analyze that later to see if something is awry.
I'm just saying... I have somewhat of a new found respect for log files!