Any justification for separate database
To add to Erik's, here are three advantages to separating the databases:

1) Upgrades.  When we are working to upgrade Civi, I know that the website side of the house is largely unaffected (not completely unaffected, but mostly), and if something in the upgrade goes awry, I'm only having to deal with the Civi side of things (especially in the event of downtime).

2) Restoring backups.  Should you need to restore a backup of your website (like we did last year), you are only losing the web changes made since the backup, not the Civi changes as well.  The same is also true if you need to restore the Civi backup -- you then don't lose the website changes made since the restoration point.

3) Multi-site installations. See the documentation here: Multi Site Installations